Grammas are a good fish and usually peaceable. However, they can be territorial and they will defend their home. Usually they would just chase the clowns away, and not really hurt them.
A couple of things you can try. Rearrange the rock work and then reintroduce the gramma in a few days. The new layout makes it seek a new place to call home.
Your water temp is a little high. Often a slightly cooler temp, maybe 78, will keep the fish less aggressive.
When adding new tank mates, choose ones similar in size or larger than the gramma.