A jury deliberated for three hours before finding James King not guilty of trying to pervert the course of justice (the equivalent of obstruction of justice in the U.S.).
The singer-songwriter's husband, Blake Fielder-Civil, has admitted beating up King in a barroom fight in 2006 and then offering him $400,000 to keep quiet about it.
King, who suffered a broken cheekbone, said he was intimidated into withdrawing the assault claim.
Three other men also pleaded guilty to involvement in the plot. They and Fielder-Civil all face jail terms when they are sentenced later.
Winehouse, 24, and Fielder-Civil, 26, were married in Miami in May 2007. He was arrested in November and has been in jail ever since.
She has become an international star since releasing the Grammy-winning album "Back to Black" in 2006. But her music has been overshadowed by reports of drug use, her run-ins with the law and tempestuous relationship with Fielder-Civil.
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