A secret shipment of high explosives to Libya in 1977 was worth "a small fortune" to ex-CIA agent Edwin P. Wilson and was to be transported "as clandestinely as possible," Wilson's former business partner said in a tape played by prosecutors in Wilson's conspiracy trial here today.

The tape, recorded by one-time Wilson associate Reginald Slocombe, contained instructions to employes at a Houston freight company owned by Wilson and Slocombe for illegally shipping 20 tons of C-4, a plastic explosive, from Texas to Libya.

Federal prosecutors completed their case late today after three days of testimony suggesting that the explosives, flown by chartered jet to Libya in October, 1977, netted substantial profits for Wilson. The defense is scheduled to begin its case Wednesday morning.

Defense lawyers maintain that Wilson, 54, was more patriot than profiteer, and that the ex-spy was using business activities in Libya as a cover for intelligence-gathering beneficial to the United States. U.S. District Court Judge Ross N. Sterling allowed the playing of today's tape over strenuous defense objections.

"It's stuff called C-4 and it is not supposed to be exported, so this shipment becomes a very hot shipment," Slocombe said on the tape, recorded at the Washington office of Wilson's company, Around the World Shipping. Slocombe said he was taping the instructions because "the office is bugged and I feel my home is bugged also."

". . . The reason Ed wants to do it is there's a small fortune involved, profit-wise," Slocombe continued. "The main thing is, it has to be done as clandestinely as possible. Therefore, I don't want to talk about it on the phone . . . . "

Later on the tape Slocombe issued directions to Edison Frazier, a company employe in Houston, for using code to set up contacts from pay phones to discuss progress on the explosives deal with Slocombe.

Frazier acknowledged on cross-examination today that he dealt mainly with Slocombe and had met Wilson only once. He testified he assumed that references to "Ed" on the tape meant Wilson.

Prosecutors have sought to show through a series of witnesses that Wilson masterminded the explosives deal from his base in Tripoli.

A California explosives dealer, Jerome S. Brower, testified that Wilson asked him to purchase "as much as I could get" of the C-4 and then transferred $588,000 from Wilson's Swiss bank account to a second Swiss account belonging to Brower to finance the deal.

Wilson's former financial manager, New York lawyer Edward J. Coughlin, testified on Monday that that transaction was funded by nearly $1 million deposited in Wilson's Swiss account by the Libyan regime of Col. Muammar Qaddafi in 1977.

Another former Wilson employe, ex-Army bomb expert John Heath (whose code name in the Wilson empire was "Silver Fox"), said on the witness stand that he had been recruited by Wilson to train Libyan terrorists to use C-4 explosive to build booby traps such as exploding lamps.

Heath testified that he received a flurry of telephone calls--"five or six calls within a half-hour or 45 minutes"--last month from Wilson from his jail cell in New York discussing Heath's plan to return to the United States and talk to federal prosecutors. "He told me he could not advise me against it because it would be an obstruction of justice, but if I talked to Barcella, he [Barcella] was dead and so was I," Heath said, speaking of Assistant U.S. Attorney E. Lawrence Barcella.

Prosecutors fought in a closed hearing last week for Judge Sterling's permission to inform the jury of an alleged plot by Wilson while in jail to assassinate Barcella and another prosecutor as well as five government witnesses, according to one source.

Sterling, who has had the jury sequestered since Friday, has refused to make public any portion of last week's long pretrial record.