Clowns play with sand. Most do it, not an indicator of breeding activity.
Clowns are more likely to breed if they feel safe and low stress. They spawn more readily if they are the only fish in the tank; they can spend so much time watching other fish that they never get around to thinking about the next generation of clowns. Wilkerson is a good reference, but it is really very general when it comes to breeding. You might also want to check out Conditioning, Spawning, and Rearing of Fish with an emphasis on Marine Clownfish by Frank Hoff. Or check out Woodys thread on it here at RS