Under siege by the feds, the former business partner of Steven Seagal will admit he tried to shake down the action film star, authorities said yesterday. Film producer Julius Nasso was charged last year with conspiring with the members of the Gambino crime family to extort $3 million from Seagal. Yesterday, Assistant U. S. Attorney Andrew Genser disclosed in Brooklyn Federal Court that the feds had reached an "agreement in principle" with Nasso to plead guilty next Wednesday. Under the deal, Nasso would serve one year in jail. He would have faced five to eight years in prison, if convicted of attempted extortion charges. "He just wants to put this behind him and continue with a fruitful, productive life," said his lawyer Robert Hantman. Seagal testified about the shakedown last February at the racketeering trial of Gambino boss Peter Gotti. He described a now infamous sitdown dinner at the landmark Gage & Tollner restaurant in Brooklyn at which a Gambino capo warned Seagal to make movies with them and Nasso. It is unclear what impact the guilty plea will have on Nasso's $60 million civil lawsuit against Seagal. "Mr. Seagal is pleased justice has been served, that he has been totally vindicated," Seagal's lawyer Martin Pollner said last night. The prosecutor also revealed that Nasso's brother, Vincent, has pleaded guilty to one count of wire fraud in a mob scheme to award his company a lucrative prescription drug contract for the International Longshoremen's Association. Vincent Nasso will face two years in prison, a source familiar with the deal said.