Katharine Hepburn in a role she can lavish her wit on

The West Side Waltz Comedy by Ernest Thompson. Starring Katharine Hepburn and Dorothy Loudon. Directed by Noel Wilman.

Katharine Hepburn fans can relax. And playwright Ernest Thompson presumably can do likewise. Mr. Thompson has written a slight but serviceable comedy with the kind of starring role on which Miss Hepburn can lavish her radiance, wit, and forthrightness for as long as she cares to play it. The comedy is called ''The West Side Waltz'' and - after touring for five months last season - it is now scheduled to tenant the Ethel Barrymore Theater until March 13.

Miss Hepburn portrays a crusty, reclusive former pianist and teacher who fights her loneliness rather than admitting it. Widow Margaret Mary Elderdice is not one of the truly needy as the Reagan administration defines them, but her dinner may consist of a can of tunafish. More to make music than for sociability , Margaret Mary tolerates the company of her spinster neighbor, violinist Cara Varnum (Dorothy Loudon). Cara is one of those self-consciously anonymous people who yearn for recognition and approval.

In an uncharacteristic gesture that betrays her underlying need for companionship, Mrs. Elderdice advertises on a neighborhood bulletin board for someone to share her apartment. The ad is answered by Robin Bird (Regina Baff), a kooky would-be actress from Brooklyn. ''The West Side Waltz'' thus deals with not one, but two, generation gaps.

From human ingredients similar to those more skillfully employed in his ''On Golden Pond,'' Mr. Thompson has fashioned a humorously slender tale of loneliness, relationships, and the challenge of aging. Although ''The West Side Waltz'' sometimes makes its way as much by contrivance as by logic, its heart is in the right place. The results are at times touching and on occasion very funny as the two musicians and the young newcomer react to and upon one another.

Whatever Margaret Mary's method of locomotion - from one cane to two, to a walker, and finally to a wheelchair - Miss Hepburn commands the situation. She softens the old woman's sharp tongue and sharp edges with her own warmth and spirited humor. Miss Loudon plays the longsuffering duet partner with a whole repertoire of pained expressions and with the cringing stance of one perpetually braced for the blows of outrageous fortune.

As the gritty but not insensitive girl from Brooklyn, Miss Baff grapples gamely with the somewhat unaccountable shifts of motivation and behavior Mr. Thompson requires of her. Don Howard makes a pleasant appearance as the lawyer who comes into Robin's life. Mr. Thompson also employs the handiest comic fixture of New York apartment-house comedies, the garrulous superintendent-handyman, in this case a Romanian. David Margulies acts him handily.

''The West Side Waltz'' requires six scenes, covers several seasons, and calls upon composers ranging from Strauss, Mozart, and Czerny to Chopin and Jimmy McHugh. David Krane arranged the musical excerpts. The production was costumed by Jane Greenwood and lighted by Thomas Skelton. Noel Wilman directed. A bust of Beethoven gazes sternly out from high atop a shelf in Ben Edwards's setting for Margaret Mary's comfortably faded living room. Beethoven's lack of amusement wasn't shared by the other spectators the night I visited the Ethel Barrymore. They were clearly delighted.