The man who brought chills to millions of summer vacations with Jaws and The Deep has returned. But this time out, Peter Benchley has jettisoned the oversexed surf-and-turfers in favor of Timothy Burnham, a fortyish journalist turned speechwriter whose only obsession is quoting the wisdom of Samuel Johnson, as in ''No man but a blockhead ever wrote, except for money.'' Burnham writes not only for money but for President Benjamin T. Winslow, bullying, foul-tongued and Johnsonesque (Lyndon, not Samuel), and the assignments are rarely more demanding than ''Representative Whipple has told me a great deal about the fine work you ladies are doing in the Leesburg Macrame and Dialysis Society.'' The President barely knows the name of this second-string hack until a bureaucratic glitch awards Burnham a ''Q'' clearance to receive atomic secrets. Though he has no idea what to do with them, or with the accompanying paper shredder, he soon attracts the attention of Soviet spies, jealous White House insiders and, worse, the President, who makes him a trusted adviser. Benchley's story embraces the debate over invading Honduras (Ronald Reagan's earlier incursion into Nicaragua having failed) and a yachtload of American homosexuals who threaten to blow up a Soviet supertanker in Cuba. But all that is mere backdrop for a mordant overview of Washington props and icons: a Cabinet Room table has buttons underneath marked ''Coke, Tab, Fresca, Pepsi, Coffee, Tea.'' When told that he is heading for the wrong aircraft, the President roars, ''Son, they're all my helicopters.'' At the end, ''Q'' Clearance dangles an intriguing question: Where did a onetime spinner of sea-horse operas learn to write comedy? Perhaps from his grandfather, Humorist Robert Benchley, or from his father, Novelist Nathaniel, or even from the exasperating Johnson (Lyndon, not Samuel), for whom young Peter once worked as a White House speechwriter. In any case, this Benchley's latest effort contains some memorable slapstick. When Burnham splits his pants on the way to his first audience with the President, he solves the problem with his desktop stapler, leading to some hilariously complicated results. And the novel offers an impudent phrase or two. A pair of White House aides in a hallway ''looked at each other's shoes, the way important people always do when they exchange profundities on the hoof.'' Burnham's fellow atom-secret keepers, he explains, are into ''research, development and construction of the flatware for the Last Supper.'' Power in Washington meant ''entree (at least once) to redoubtable Georgian manors where the glassware tinkled with clarion clarity and no wine was served before its time.'' Beneath these cracks there sometimes appears a healthy anger with almost as many teeth as Jaws. Not a bad attribute for a for a man who wants to exchange wet fantasies for dry humor. As Johnson (Samuel, not Lyndon) once observed, ''Of all the griefs that harass the distrest,/ Sure the most bitter is a scornful jest.''