THERE IS A KIND of fusty, conventional prejudice in the matter of obituary commentaries, a constraint that works in favor of those whose contributions of one sort or another have been not only distinguished but conspicuous and inarguable over a period of many years. And the fallacy of it is nowhere more powerfully demonstrated than in the life Marion Clark, whose career with this newspaper was no less distinguished, in a very real sense, for having been tragically cut short. The part of it that we know of with certainty was not only substantial but of such style and quality as to leave no doubt of its promise - of what surely would have been. Marion Clark was only 35 when she was killed over the Labor Day weekend in a grim, freak accident. But she had already established herself as an editor (of potomac Magazine) and as a reporter (of the Wayne Hays/Elizabeth Ray story), and she was about to enter upon a new phase this week as editor of Potomac's successor, The washington Post Magazine, which will begin publication on Sunday. In professional terms, she was not just on her way; she had long since arrived, as an established star whose energy and hard work and great gifts helped shape this newspaper for the better and won her the profound respect of all who worked for it and with her.

It was impossible to be a colleague of Marion Clark's without having the sense of being, at once, a friend. A sligghtly madcap friend, who would stand in line with the kids at an employees-and-family jamboree in celebration of The Post's 100th birthday for her turn at the pinball machines . . . who would joyfully improvise a spontaneous bowling contest down the long corridors of the newsroom late in some crisis evening, using oranges provided as part of the repast for the overtime help . . . who would argue passionately, and not without some impressive credentials, for the assignment to cover an ascent of Mt. Everest . . . who wrote with the intensity of a gourment (of just about all of life's joys, including food) of the Ineffable Oeuf and of five exotic recipes for salvaging the hard-boiled remnants of Easter Eggs . . . and who also wrote fondly about the "snooty, all-Wasp suburb" she grew up in, and about being in love with the captain of the football team "who didn't know I existed" and about the death of Elvis Presley (her last offering) and "the way it smelled - Old Spice, sweatsox, too-sweet perfume - when we danced to 'All Shook Up' at a Teen-High."

But she was also a concerned friend - deeply concerned not so much with others' friendship for her as with hers for others, and with the myriad ways she expressed it. To be sentimental and at the same time gently cynical about all things is to be complicated, which Marion Clark surely was; but that's simply another way of saying that she had, to borrow Robert Frost's phrase, a lover's quarrel with her world - the popular culture she grew up with. Her former editor, Shelby Coffey, spoke in an interview after her death of the "self-mocking romanticism that made her, as writer and woman, an innocent treasure." That's exactly what she was - and why she will be so sorely missed.