Scuttling crustacean (4)
I believe the answer is:
crab
'scuttling crustacean' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for crab that I've seen before include "Seashore crustacean" , "winch" , "Nautical nipper" , "A crustacean; type of apple" , "Sideways scuttler" .)