To honk a clown's nose simply melee it. You will hear a honking sound after you hit them.
These are uncommon infected, only found in the Dark Carnival campaign, very few in number. Just be on the lookout for these during this campaign, you will get it in time. It will count or you honk the same clowns nose more than once, but they will die after 2 or 3 either way.
Also, you can't honk their noses if they're dead, so I recommend playing with a group of friends online or possibly killing the AIs
(which really isn't worth it in the long run) so you have a better chance of finding a live clown to honk of the noses.