"JOURNALISM WAS marvelously seductive fun," Gregory Mcdonald has said of his seven years as a reporter for the Boston Globe. And McDonald has marvelously seductive fun with journalism in Fletch's Fortune (Avon paperback original, $1.95).

Fletch - anyone with a name like Irwin Maurice Fletcher (try I.M. Fletcher and it sounds like a self-important pronouncement) would adopt a nickname - breezed upon the crime scene three years ago and brought McDonald an Edgar award for the best first mystery novel of the year. Fletch is also a former newspaperman, and an unmanageable free spirit, cocky, irrevernet, and thoroughly engaging.

In Fletch's Fortune, the ex-reporter has returned to the Riveria to enjoy the fruits of his earlier capers (chronicled in Fletch and Confess, Fletch ). After a nice morning frisking on the beach with the next-door neighbor whose husband sleeps late, Fletch finds two men waiting for him in the living room of his Italian villa. CIA, they tell him, and they want him to bug the rooms at the American Journalism Alliance's annual convention. When he refuses, they remind him of the tax returns that he never bothered to file.

Before Fletch reaches the conference center near Washington, Walter March, the press tycoon, has been found stabbed in the back with a pair of scissors. A singularly appropriate means of dispatch, it turns out, for March was a ruthless man who back-stabbed quite a few of his colleague many of whom consequently had good reason to wish him dead.

The seductive fun of NFletch's Fortune is matching the fictional characters with familiar bylines or with well-know TV personalities. Mcdonald can be as irreverent as Fletch when it comes to puncturing journalistic egotism and posturing. With varying degrees of maliciousness glee, and fairness, he spears the avuncular, conscientious television anchorman, the syndicated columnist whose snide insinuations pass for investigative journalism, the tough, earthy woman reporter, the cigar-chewing humorist, the feature writer who bleeds with the people, the net-work pontificator who delivers 90 seconds of polysllabic commentary, and the woman who has made it to the top of TV news.

The bugs planted in the conference bedrooms give Mcdonald an ingenious device for eavesdropping on conversations. there is a hilarious moment when the conference planners try to decide what to do with a President who is to speak at the convention and who doesn't golf. Fill the swimming pool with catfish and give him a net? Organize a soft-ball game?

Fletch, with the habits of good reporter, does his research on the people with whom he is dealing, and a prosaic entry in Who's Who gives him the clue to March's murder. The solution is a bit contrived and, at times, Fletch strains so hard to be bright and smart-alecky that you want to say: "Down, boy, down." But most of the time he's entertaining and great fun.