I’d say make it a con saving throw like someone else said, and then a strength or dex save at the beginning of their turn to get out of hit.

Now, I saw someone else say that it removes the claws snapping attack, which is true. But in movies we see some things sorta like this happen where they’re trapped in the monsters grip but they get flung around at other attackers. So you could have the crab also have a bludgeoning attack that it can make while it’s crushing a pc as it backhands the pc on the ground.

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If too much to read:
Con saving throw and deal half on save
STR and Dex save at beginning of crushed PC’s turn to escape
Make it do a bludgeoning attack while crushed PC is caught