i bought a blue lobster / crayfish from the lfs about 4weeks on the way home it moulted the head part and its pincer claws on both sides, (i thought it was dead)
Yes, they will be back in about a molt or two. You can already see he has them coming back.
It's just the way they grow. They continue to grow under their skin, so when they molt they expand before they regrow their shell. The claw should be noticeable next molt, and back to full size the next molt after that.
I've had crays loose ALL their limbs and make a full recovery.