Headless rock fish (4)
I believe the answer is:
hake
'fish' is the definition.
(hake is a kind of fish)
'headless rock' is the wordplay.
'headless' means to remove the first letter.
'rock' becomes 'shake' ('shake' can be a synonym of 'rock').
'shake' with its first letter taken away is 'HAKE'.
(Other definitions for hake that I've seen before include "Food fish related to the cod" , "Swimmer" , "Cod-like fish" , "Type of fish" , "Food fish; rhymes with create" .)