I seriously doubt they would fight over a anen with a clown, but I couldnt be 100% sure of this. You could try it.
If they did battle, the clown would easily drive off the shrimp and they are fast enough to get away no problem. Plus they will live near ANYTHING. Polyps, gorgonian, even mushrooms. So chances are if a clown has a anen they will pass this over to occupy something else.
And VERY important. They PREFER to live in groups. Buy at least 2
Plus they do not need an anen to live in; they just like it.