dmck,
I don't have any first hand experience with this happening, but I took a look at what Daniel Knop had to say about mantle lacerations in his "Giant Clams" book. He attributes most to either parasitizing crabs or harassment by fish. If the culprit can be removed and/or the clam protected from further damage, an otherwise healthy clam can often heal itself.
What fish and crabs do you have in your tank? How big is the clam and how big is the tear in the mantle? Up until this point, had the clam been displaying full mantle and coloration every day, and reacting quickly to sudden movement near it?
Cheri