Sort of movement a crab displays (6)
I believe the answer is:
pincer
'crab displays' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Perhaps there's an association between them I don't understand?
I don't understand how the rest of the clue works.
(Other definitions for pincer that I've seen before include "Front claw" , "Crab's claw" , "Crab's nipper" , "Grasping limb of a crustacean" , "Claw, nipper" .)