Dinner exactly at six

''Dinner will be exactly at 6:00, so please don't spoil your - ''

''Since when have we had dinner exactly at any time.'' Alexander continues to nibble cheese. ''One night its 8:00, the next 5:591/2, another night we don't actually sit down till 11:00. And it's never ready when you say it will be.''

''For 20 years I served dinner exactly at 7:00 . . . Or at least I tried to. . . . But now our lives are less structured - and more interesting, don't you think?''

''Sure. . . .''

Wistfulness and complaint still lurk behind his blue eyes. Yet this 16 -year-old, whose tousled tow hair is always a fraction too long for school authorities, is supposed to be the blithe spirit. He disdains homework, schedules and bedtimes, and used to write poems and funny stories. Alas, he seems to write none now. . . . His childhood charm and candor have been replaced by sullenness and taciturnity.

''So what's eating you?'' I hand him a drawerful of silverware to set, though I'm not sure how many will be around tonight's table. Fortunately Cammie is kneading a mass of bread while studying for her mycology exam.

''When I was staying at Great Aunt Emma's this summer,'' Alexander finally begins as if begrudging every word, ''or at anyone else's house for that matter, dinner was always on the table at 7:00. Lunch was at noon, with the best desserts. Breakfast was ready when I came down, and the toast was never burned. And I wasn't constantly being bugged to clean up.''

''How nice. . . . Now could you please peel these onions, since you can peel without weeping?''

''So can Cousin Henry.''

''He's writing against a deadline on his thriller. So am I. But I have ten pounds of squid to clean first. I'll mend your jeans if you'll sweep this crunchy floor.''

''Later. I hate sweeping floors.''

''Then do your homework now.'' Terrible grades lately, no motivation, no -

''I thought Cousin Henry hated to eat so early.'' Alexander adds that he's too hungry to wait till 6:00 anyway.

''Great Aunt Emma has invited us all to a new play. Her way of supporting a struggling experimental theater.'' I don't let on that Great Aunt Emma and I connived to plan some activity that specifically involved Alexander.

''Drama's not my bag.''

How exasperatingly negative he's become.

''Maybe not. . . . This isn't a play for children. You can get to bed on time instead of going out. But there's a copy of the script on the piano.''

As I clean the squid - how can I possibly clean it and boil and marinate it by 6:00? - I hear Alexander and a visiting Malay poet, Mohammud, reading the script aloud in the living room. Sometimes they're serious, sometimes they ham it up, and now they're speculating how they'd play the various roles. I muse on how poorly I fill all my roles, including that of mother. Easier to fill that of writer. . . . Though not in the house. . . .

Dimitri, an exchange professor of physics, stops in as it gets dark: Would I translate his article on transmutation of metals? I guess everyone will be here for supper if I can just get it ready by 6:00. At the Tolstoy estate, Yasnaya, Dimitri says guests often came for a holiday and forgot to go home. A rich feast for young Leo, I think. Would that out of our own rich chaos, something--

''Does it really bother you,'' I ask Alexander, ''that we have - seemingly - so little schedule?''

''Just that other people do.''

I pause in the translation for Dimitri to answer the tenth phone call of the hour, then try to sew Alexander's torn jeans. Would that I could repair as many torn lives. Perhaps sew the missing button on my own. . . .

''You should write a spy story,'' Alexander suggests, gnawing a carrot.

''When would I find time? You write it.'' I return to the translation.

''Pretentious as it may sound - '' I begin when Alexander transects the study where I am briefly anchored, ''we are all working artists here. . . .''

''Not I!''

''Yes, you too. We each have to create some internal structure, and encourage it in each other, and we do have schedules, even if they sometimes sag, and each one's doesn't always coincide, and - ''

''Since you obviously won't have supper ready at 6:00, may I borrow your typewriter?'' A long time since he's troubled himself to type his homework, so I unroll Dimitri's metals in metamorphosis, and go chop the cabbage so Cousin Henry can make the salad dressing.

Cammie complains her bread is slow to rise. Cousin Henry makes the salad but wants time to practice. We are getting late . . . and later.

Great Aunt Emma phones: a flat tire. Alexander bicycles off to change it, thus winning the right to drive her 1938 Rolls-Royce here. But now it's too late for dinner before the play - and Great Aunt Emma mustn't miss--

''Don't worry about me,'' she says, ''I know this household. I dined at 5:00 .''

''My bread won't be baked till after the play,'' Cammie says.

''The longer the squid marinate,'' Mohammud says, ''the better.''

''I'd rather go to the play than eat,'' says Alexander.

We all cram into the Rolls-Royce, the play is a success, and we invite the whole cast back to this house fragrant with bay leaves, curried squid, hot bread. Suddenly Alexander assumes the role of host and toastmaster, and tosses off whole lines of the play with the actors as if in a badminton game. Then he grows pensive, and excuses himself for bed. But he detours to help Great Aunt Emma with her shawl, hugs her, and mumbles, ''Thanks a lot.'' Then I hear the clatter of trash cans and realize this is the first night he's remembered to put them out without being told three times. And then for another hour, as Cousin Henry and I clean up the kitchen, we hear the typewriter clicketing unevenly upstairs.