Fish stink when head is removed (5)
I believe the answer is:
tench
'fish' is the definition.
(tench is a kind of fish)
'stink when head is removed' is the wordplay.
'stink' becomes 'stench' (both can mean a bad smell).
'when head is removed' means to remove the first letter.
'stench' with its first letter taken off is 'TENCH'.
(Other definitions for tench that I've seen before include "Coarse fish" , "One in school" , "Freshwater fish" , "Swimmer" , "catch" .)