Above and beyond what Minderaser said about lighting, I would not put a Tridacna Clam of any species in such a small system. Lighting is a big issue but, ph, calcium and salinity stability can be very hard to keep stable in such a small system. In my 240 reef at home, if I lose a couple gallons of water through evaporation it does not really change my parameters. If you lose a half gallon of water if will drastically change your water parameters, in such a small system.
I am not saying it cannot be done, I just would advise against it, unless you are the most dedicated aquarist to make sure all parameters stay in accepted ranges, and then of course you upgrade your lighting.
best of luck, Rob