Did you use any kind of Krazy Glue for mounting anything right before you noticed this?
Add any new fish right before? Can it be showing a sign of aggression/dominance to another fish?
I've never heard of lockjaw on a fish. Maybe it bit down on a rusty hook! :lol2: Sorry, I shouldn't kid about things like this.
Look inside the mouth and check whether there are translucent bumps. If so, the clown has ick inside the mouth and the gill area. It's very very bad sign. Treat him immediately. Chances of survival at this stage is not high.