New York Music Critic Stephen Fauldgate (George Rose) is a figure of fun in ''Beethoven's Tenth,'' the new Peter Ustinov fantastical comedy at the Nederlander Theatre. Fauldgate also gets the action going, which proves that critics can sometimes serve a useful purpose. In this case, Fauldgate proves a functional means to a fanciful end. Here's how the idea for the play originated: ''It was (the) feeling that immortals are just out of sight, out of earshot, which led me to imagine the effect on a family obsessed with somewhat mundane problems of a visit of just such a great figure of the past, not in the form of a ghost or spirit, but in a remarkable palpable and indeed boisterous form,'' writes Ustinov in the Nederlander playbill. ''Stephen Fauldgate is a reputable music critic whose special study outside the contemporary scene is Beethoven. He is fastidious in his choice of words and selection of sentiment. His son, Pascal, writes facile symphonies in menacing profusion. His wife, Jessica, an erstwhile mezzo-soprano, gave up her career to make peace between the two men in her life. She is not helped much by Irmgard, an Austrian au pair whose childhood was spent with Beethoven's death mask over her cot.'' Like Fauldgate, Irmgard helps the action forward by earnestly wishing for a visitation from Beethoven, who enters the Fauldgate home following four staccato raps on the door. After these quotes from the Fifth Symphony, however, the shaggy old composer proves vague in many of his recollections. It is Fauldgate who must acquaint Beethoven with his renown, evidenced by a vast library of recordings. In one of the comedy's more touching moments, Beethoven grapples at first uncertainly with the accompaniment of ''An die ferne Geliebte'' ( To the distant beloved), sweetly sung by Jessica (Mary Jay). Shabbily dressed and untidily bewigged, Ustinov's Beethoven proves a formidably puckish revenant. The exalted guest spends his time eating, opining, and recollecting the existence that ended for him in 1827. Recalling his own unhappy relationships, Beethoven nevertheless undertakes to be a kind of family-counseling Mr. Fixit for the Fauldgate household. He is comically larger than life - turbulent, truculent, earthy. He can also on occasion be more than a little moving. Mr. Rose plays Fauldgate with sharp comic observation, perceiving the intellectual substance beneath the critic's self-importance. Adam Redfield performs capably as the young would-be composer, one of whose biggest moments occurs when he tells Beethoven the Ninth Symphony is too long. (Beethoven almost agrees.) Gina Friedlander's au pair girl is Teutonically ardent. — Beethoven's Tenth. Comedy by Peter Ustinov. Starring Mr. Ustinov, George Rose. Directed by Robert Chetwyn.