For this achievement you must melee (Left Trigger if your using controller or right click with mouse) 10 clowns with any weapon. I killed them with melee hits, best safe as opposed to sorry, but am unsure if you can just hit or have to kill. Throughout "Dark Carnival" campaign, you will come across clowns mixed in with zombies. You will know it when the clowns nose honks. Here is a video guide to help as well: