The ABC Afterschool Specials have made a commendable habit of tackling difficult subjects, and the subject of a mother's death from terminal cancer is difficult enough even for an adult program. "A Matter of Time," at 4 today on Channel 7, is the kind of sensitive and straightforward drama one has come to expect of the series though grimmer and more harrowing than usual.

Karlene Crockett, a wise-looking, billowy-haired young woman, plays the teen-ager who learns early in the film that her picture-perfect mother has lung cancer and only a few months to live. At first the girl retreats into flashbacks of halcyon days; then she rebels at the trauma being inflicted on her life. Mother changes, becomes distant and brooding, and is not saved from death by any last-minute miracle.

A hospice counselor tries to advise the family on how to cope. Unfortunately, Paul Copper's script degenerates at this point into yet another seminar on specialness -- Phil Donahue meets Mister Rogers. "Jean, will you share your feelings with us?" asks the counselor. "She's so special," the girl says of her mother, and later, "Then I'll know what makes me special" and "what's special about me is me."

Kids watching this program deserve better answers than these sensitivity greeting-card cliches.

The dramatic essence of the story, though, is the facing of the mother's death, and this is handled with great finesse and intelligence, both by writer Cooper and director Arthur Seidleman. Like many of the best Afterschool Specials, "Time" was produced for ABC by Martin Tahse. It represents their best, something that can't help but be of use to children and to television.