A 1" clam does not yet posess the amount of Zooxanthalle algae to use the lighting to manufacture enough food for it's survival. You would need to dose alot of Phytoplankton(preferably DT's or another live phyto). Clams are filter feeders and cannot depend on light alone at a early age or small size.
The next problem you will have is you would overload your system with nutrients feeding your tank enough phyto to help the clam.
Now you could remove the clam into a separate container with tank water, feed the clam phyto and then return them to the tank when finished. This is going to be a large commitment, I hope you will be up to the challenge.
Next problem is the size of the tank. A 12 gallon nano, in my opinion is a very bad home for Tridacnid clams. Clams are not too hard to keep if the following rules are followed.
1) Keep the following stable.
a) salinity between 1.024 and 1.027
b) temperature 77degrees and 82 degrees
c) calcium levels between 350 and 450 ppm
d) ph levels between 8.0 and 8.2
e) a trace amount of nitrates maybe 5ppm or less
f) zereo nitrites
g) zero ammonia
In a 12 gallon tank, all the above can be very hard. Nano's are tough to keep stable. Unless your nano has a 50 gallon sump, I would suggest passing on clams for this tank.
It is not impossible to do, but it would be an uphill battle the whole way.
Best of luck, Rob