A young girl, striving to become a writer, 'holds onto love'; Hold On to Love, by Mollie Hunter. New York: Harper & Row Junior Books. 251 pp.

A bonnie young lass, she is. But beneath Bridie McShane is hardy as a freesia in March. The 1972 forerunner of this new autobiographical novel, ''A Sound of Chariots ,'' took Bridie from her fifth birthday to her 15th. Here Scotland's renowned Mollie Hunter continues Bridie's journey toward young womanhood and sought-after career as a writer.

The setting is Edinburgh on the brink of World War II, with local Blackshirts holding rallies on the Mound and khaki-clad officers and NCOs jamming the city's train station. By day Bridie works in her grandfather's florist shop, ticking off the hours until night school starts and she can begin to unravel the intricacies of language that she must master before she can become a real poet.

Or should she be a playwright? Or A novelist? Determined to make her own way, Bridie is a first-rate leading lady in a reminiscence that should draw plenty of applause from today's understudies.

The supporting cast of characters are equally appealing. With her younger brother, William, Bridie sings the old, tub-thumping tunes from Grandma Armstrong's hymnal. Her Mum, pink-cheeked as a rose, is a model of long-suffering patience. But it's Granny Armstrong who reaches out from her deathbed to implore Bridie to ''hold on to love.''

Love has come to call in the person of Peter McKinley. He's Bridie's first ''fella.'' Too protective and possessive for Bridie, as it turns out.

It could have been the stuff of mawkish midday soaps, but author Hunter never loses her sure touch with the complex character she's reliving here.

As a quiet odyssey through the labyrinth of self-exploration, this book should hold special meaning for young artists-to-be. After seizing on every experience with gusto, and then throwing out much of her verse and prose as ''high-flown rubbish,'' Bridie eventually comes to a well-reasoned reckoning of her abilities and aspirations.