Foul smell left by small fish (5)
I believe the answer is:
tench
'fish' is the definition.
(tench is a kind of fish)
'foul smell left by small' is the wordplay.
'foul smell' becomes 'stench' (I've seen this in another clue).
'left by' suggests deleting specific letters (word is left or abandoned by specified letters ).
'small' becomes 's' (abbreviation - e.g. clothes size).
'stench' with 's' taken away is 'TENCH'.
(Other definitions for tench that I've seen before include "something fishy" , "One in school" , "catch" , "Coarse fish" , "Fish of the carp family" .)