What kind of clam? If you're talking about a tridacna (giant) clam, most keepers use a phytoplankton product like DTs. It's available at most LFSs. How much you feed depends on species and size. Smaller clams are more dependent on feeding, larger clams mostly utilize photosynthesis.
They don't generally respond well to target feeding. They kind of look like they're choking. Some people remove the clam to a separate tupperware container, add the phyto, and put the clam back in the tank when the water turns clear. You can also put the stuff right into the tank.
