Fish found in rock, missing head (4)
I believe the answer is:
hake
'fish' is the definition.
(I know that hake is a type of fish)
'rock missing head' is the wordplay.
'rock' becomes 'shake' (synonyms).
'missing head' means to remove the first letter.
'shake' with its initial letter removed is 'HAKE'.
'found in' acts as a link.
(Other definitions for hake that I've seen before include "Food fish related to the cod" , "Type of fish" , "Swimmer" , "Common food fish" , "Cod-like fish" .)