It depends. My clowns are aggressive with my anemone crab every now and then, epsecially if they are about to or just recently spawned, but they've learned to get along. If your clowns are really aggressive I wouldn't even try it (with maroon clowns, for example). If they're semmy aggressive, a crab is a better idea than a sexy shrimp.
keep in mind that sexy shrimp don't "need" anemones, they can also use a frogspawn, hammer or torch coral. Also, one time my female clown was very aggressive and kicked my anemone crab out, he hosted with a SPS colony for weeks and slowly ventured his way back into the anemone and now they get along.