Sideways-walking crustacean (4)
I believe the answer is:
crab
'sideways-walking crustacean' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for crab that I've seen before include "Sideways scuttler" , "Seashore creature" , "Seashore crustacean" , "Nautical nipper" , "A crustacean; type of apple" .)