Never had a marine betta, but I think it is technically in the grouper family, as are assessors. Assessors do not get very large, around the same size as a royal gramma, and share very common habits. I know if my royal gramma could fit a clown in its mouth, it would eat it, and I've heard that marine bettas will eat whatever fits in their mouths, so I would avoid it if it looks like it can eat your clown.