Thanks for the input - all is well. I should have said that the urchin wasn't normally on it and the pinching had been going on for weeks. While I had put the clam in more-or-less the middle of the tank (width-wise) it had moved itself close to the glass. Since it put itself there I assumed that's where it would do best.
I moved the clam over a week ago back to the middle and pushed it deeper into the sand. It now opens fully each day and no more pinched mantle!
I guess like people they don't always make the best choices for themselves. Thanks for the input!
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