Yes you can do a fw dip on a clam for up to 15 min. I only do this for treatment of pinched mantle. May I ask why you wish to do a fw dip? It is very stressful and should only be done as a last resort treatment.
Do you have a distressed clam>
If so what are your params, fish and inverts, lighting, what do you suppliment and can you post a pic of the clam in question?