I have seen it done in very large tanks, but I wouldn't recommend it, depends on the type of clown. Ocellaris clowns MIGHT, emphasis on MIGHT be ok as long as they've got enough space in there and even then I'd say no. If you wanna keep something with seahorses I've seen them best kept with shrimp gobies, jawfish, some filefish, pipefish and peaceful inverts. Hope this helps.