Is the foot attached beneath the sand?
I've heard of some folks just wiggling their smaller clams loose, but this seems to me like it would put a lot of stress on the byssal organ.
Definitely don't want to damage the byssal organ so; I've been taught to try to tip the clam to one side a little bit, and make a nice
really clean (surgical clean!

) cut to severe the byssal strands as far away from the byssal organ/clam body.