That is not pinched mantle. The clam is gaping. That is fairly normal with Crocea's after they have been shipped. Just keep an eye on it and make sure it doesn't start to look worse. Should clear up in a week or 2 as long as nothing stresses it more. Just to make sure you may want to take it out and check for pyramid snails.
I bought a clam about a month ago and it looked the same way. It even had the hole. I wondered the same thing but it appears to be doing fine now. said: