I have a pair of Clarkii spawning now, and they host in a Haddon's anemone. I think the anemone can give a pair a sense of security, and may help SOME reluctant pairs spawn. I know for commercial breeding, bare tanks with pots or tiles is the norm, mainly for ease of cleaning, but I like to watch the relationship between the fish and nem. So, clowns will breed with or without an anemone, I think either way it is important have well conditioned clowns on a regular day/night cycle that are comfortable with their surroundings.