It's not easy, but possible. There's many places aquaculturing clams, and most are just in a netted area in the ocean, or a facility where new ocean water is consantly pumped through. I don't remember all the specifics, but it's a very complicated task to breed clams, and definitely not something for a simple hobbyist IMO. Stress is often used to get the clams to ovulate or sperm, or the eggs are extracted with a needle directly from the clam.