Thank you. I am sorry to hear about your clam and the hitchhiker. Like Ohiomom said, if it is responding to stimulus then there is a chance it will make, but 1/2 a mantle is quite a bit of destruction. Nass' usually will leave a live clam alone. Does it retract the mantle when you pass your hand over the top of the tank?
Also, did you get the hitchhiker out of there? If not, you should consider putting something over the top of the clam (like very clean soda bottle half with holes cut in it). This will still allow for flow but add protection until it can regain strength.