There is possibility that the clown was kill by something else instead of the anemone. What stickleback observation is true. When the anemone got hold of some food, the mouth will be all puff up to swallow the prey, not at all like the mouth in the picture.
I know Haddoni and LTA eat clowns but these are clowns just release into the tank. Never after they live in the anemone for a while. I have never heard of Gigantea eating A. percula other than this one.