It seems to be the case that certain types of clam reduce nitrates though there does not appear to be much information on this. Forum discussions from hobbyists suggest that there are people using clams for this reason in aquariums but most seem to be of the opinion it does not make a massive difference.
If you are interested in trying you could acquire a test kit if you do not already have one and monitor your ammonia, nitrite, and nitrate levels with the introduction of clam(s) and see what effect this has.
I would not rely on this as your only form of filtration as it may not be reliable and you also run the risk of a nitrate crash if your clam dies.
Also clams are known to bury themselves away in substrate and without careful monitoring it is possible for them to pass away without you noticing, being left to pollute your tank.