If flames and clams could coexist in the same tank without any concerns that the day least expected, you little angel will do that first nip... that would be the perfect stress free reef tank...Mhhh, then again, few aquarists can make the claim that their reef has never ever, given them any headaches!
Both animals are beautiful, and deserve the best. For me (and for me only, everyone can have their own opinion) taking chances by putting them together is just not worth it. I don't mind at a flame nipping here and there, but not on a clam. A clam could be in serious trouble if harrased by a persistent tankmate (be it fish, crab, shrimp, etc).
Just my opnion and $.02
HTH
Omar