Approach Willie as he leans against the stage with a groupie nearby. He will tell Trevor to leave before he beats him up. Go ahead and lay him down with a quick series of punches and kicks as Trevor. If you want it over quickly, then use Trevor's Rage ability to down him quickly (just a hit or 2 will do it).
Once Willie goes down, a blue blip will show on the minimap. This is the Gold tooth you are after. Grab it and quickly leave the club. With a little luck you won't have a 1 Star Wanted Level to shake off.