A self-described Chicago mobster and murderer came out of hiding as a government witness at a sentencing hearing before a federal judge today and named Joseph (Joey the Clown) Lombardo as the man in charge of authorizing killings for the Mafia here.

Witness Frank Culotta, 44, said he was told by his immediate superior as long ago as 1975 that he should "make sure you clear it with Joe Lombardo " if Culotta wanted anything done in Chicago, "especially hits."

Lombardo, 54, was portrayed today as one of the highest-ranking "crew bosses" in the Chicago underworld who could give orders even to insurance millionaire Allen M. Dorfman in peremptory fashion.

Dorfman was shot and killed in a gangland-style execution Jan. 20 while awaiting sentencing with Lombardo, Teamsters union President Roy Lee Williams and two other men. They were convicted in December for conspiring to bribe then-U.S. senator Howard W. Cannon (D-Nev.).

The focus on Lombardo sharpened this morning when FBI agent Arthur Pfizenmayer testified that he overheard Lombardo giving Dorfman orders in a telephone conversation April 9, 1979, and summoning the Teamsters pension fund consultant to a Chicago restaurant, where Dorfman was sternly lectured by Chicago underworld boss Joseph Aiuppa.

Pfizenmayer said he rushed to the restaurant, sat at the bar about 25 feet from the men and watched for about an hour and a half while Aiuppa occasionally pounded a table and Dorfman listened uncomfortably, speaking up, it seemed, only to answer questions.

Prosecutors offered no explanation for the heated session, but authorities have said they believe that Aiuppa was annoyed with Dorfman for having undertaken to bribe Cannon on behalf of the Teamsters union, without having cleared it with Aiuppa. The bribery deal was never consummated.

The testimony came in the second day of hearings at which prosecutors are trying to persuade U.S. District Court Judge Prentice H. Marshall to impose stiff sentences on most of the defendants.

Pfizenmayer learned of the meeting because he was the field agent in charge of monitoring an FBI wiretap on Dorfman's office phones when Lombardo called. On the FBI recording, Lombardo was heard telling Dorfman to "listen" and giving him instructions on how to get to the restaurant.

Pfizenmayer, who listened to hundreds of hours of conversations during the year-long surveillance, said he was struck by the phone call. "Nobody told Allen Dorfman what to do except Joseph Lombardo," Pfizenmayer said of all the telephone talk he monitored.

Culotta began his testimony by describing how he had turned government informer in 1981 after a life of crime that included two murders, knowledge of two others, "15 or better" armed robberies, uncounted burglaries, about 25 cases of arson and two instances of perjury.

He said he is serving an eight-year sentence for various crimes and has entered the government's witness protection program because he is afraid that Lombardo and Anthony Spilotro of Las Vegas, another gangster to whom he reported, want to kill him.

"I was considered part of his Lombardo's crew," Culotta said, adding that he also worked in Las Vegas for Spilotro, reputedly overseer for Chicago crime family holdings in casinos there. Culotta said Spilotro told him at a 1975 dinner in Las Vegas to "clear it with Joe" for anything Culotta wanted done in Chicago, "especially hits."

Denounced by Lombardo's lawyer as "the vilest piece of scum that ever emerged" from his West Side neighborhood, Culotta calmly said he once tried in vain to get Lombardo's approval to kill someone Culotta thought responsible for a number of melees in a discotheque he ran.

Culotta said Lombardo told him to try giving the man a message first and just "break his hands, break his legs."

Culotta said another mobster once asked Lombardo's permission to kill Culotta after a fracas. He said Lombardo told the man, "I'll let you break his head," and then ordered Culotta not to fight back while the man broke a bottle over his head and bashed it several times with a brick.

"I started punching back, but Joe Lombardo told me, 'Don't punch him,' " Culotta testified. "I knew then that I had to take a licking because I knew I couldn't win."