An actor playing a doorman on tonight's "Shaun Cassidy Special" asks the star, "Hey, when's this gonna be on?" It was a good question. The program was taped in 1981 but sat on an NBC shelf until now. It's so old that one of the guest stars is the late Jack Albertson.

Basically a harmless hour of nearly definitive fluff, "A Shaun Cassidy Special"--at 8 on Channel 4--showcases a young performer bereft of musical initiative but full of the old charmeroo. Young Cassidy is intensely personable and unassuming, the kind of lad fathers probably think they can trust their daughters with, even though the daughters may have other ideas.

During the program, he is seen shaving, petting his dog, crawling through a tunnel to make a dramatic stage entrance in Anaheim, chatting with Loni Anderson, and singing several songs in concert, including his first biggie, a remake of "Da Doo Ron Ron," assisted by The Crystals who had the original hit.

Cassidy is a respectably good light comic actor who persists in the pursuit of a singing and composing career. Immediately following a sober soliloquy about the "influences on my music," the special cuts to a Cassidy performance during which, while wearing what looked to be Sandra Dee's old satin pedal pushers, he leaps, wriggles, bumps and grinds about the stage, his baby fat dew-dropped with Sparkletts sweat, while what sounds like a million squealy-boppers shriek with delight.

At such moments you get the feeling a lot of people close to Shaun Cassidy have humored him for too long about the significance of his potential contribution to the world of song.

NBC had big plans for Cassidy way back in its pre-Silverman era. But all the plans netted was this long-shelved special. In that context, Cassidy's boyish eagerness on the program becomes poignant. Meanwhile, a new remodeled version of Cassidy will appear earlier tonight on the syndicated "Solid Gold" show on Channel 20. Worse things could happen than that this nice young man were to become a great big star.