In "Gracious Living," which opened yesterday afternoon at the Eisenhower, an experienced, often successful writer of light comedy is trying to tap that elusive vein. But Samuel Taylor's latest light entertainment is both overweight and underfed.

Yielding to none in appreciation - and hunger - for frivolity and whipped cream in a portentious age, I found myself saying, "Okay, okay, get on with it."

The author of such light comedies as "The Pleasure of His Company" and "Sabrina Fair" here imagines a happily married couple, royalties of the screen unable to get starring jobs and down to their last Rolls-Royce. To these over-the-hill high-livers falls the chance for a London "Hamlet," in which Victoria will play Gertrude, Donald Polonius.

At London's plush Savoy, the Renshaws are shaken to learn from an insistent fan that when Donald was filming "The Guns of Kashmir," he had had a six-week fling with an adoring lady, then in the studio costume department. This liaison resulted in her son, "Sonny," now one of England's richest tycoons. When "Sonny" learned the identity of his real daddy, he determined to be worthy of so dazzling a sire.

The situation is not unpromising but Taylor works it out on too many levels. There is fun and wit in imagining how Polonius, played in the swashbuckling style of Donold's Prime, might add action to the closet scene. It is amusing to have a very good actor, which Paul Hecht is, hint at how his invisible director and Hamlet are going about their tasks.

It also is a joy to watch the inimitable Tammy Grimes tackle the snappish wife whose Gertrude gets no attention whatever. I'm one of those who delights in this American comedienne who can seem more British than the British. I like her voice, nose, style out to the house, stage center, and and ability, when supposedly talking to characters on stage, to casually turn spit out a greedy little speech.And here she is not being an inconsidedate actress.

Further, there is that divine Patricia Routledge, whose previous appearance here was the only highlight of "1600 Pennsylvania Avenue," in which, as several First Ladies, she sang duets with herself. Now Routledge is the cheery little seamstress who, having mothered his son, now wants to mother the fading film star. Routledge finds tones for her that are ridiculous endearing and enormously funny.

But "Gracious Living" has two very basic flaws. Donald is an absurd, selfish ass and one can't imagine why a wife or an agent (whom Gerald Hiken plays with genial puzzlement) would suffer him. Hecht acts him with skill but the part's contradictions are too basic to accept. The fellow has to be larger than life, outrageous yet charming.

Finally, Taylor hasn't settled on whether he is writing a farce, a comedy, a satire or a soap opera. He winds up in the latter vein. Despite handsome offers from his millionaire son, played capitally by Jamie Ross, Donald fades into True Love (the title, by the way, of another Taylor play) and we drag on to the finish line. Every time a plot twist is needed, Taylor twists a character, not our risibilities. Oliver Smith's sets are exactly right. Almost understandably, Edwin Sherin's direction cannot achieve miracles.