Small fish producing unpleasant smell (6)
I believe the answer is:
stench
'unpleasant smell' is the definition.
(I've seen this before)
'small fish' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'fish' becomes 'tench' (tench is a kind of fish).
's'+'tench'='STENCH'
'producing' acts as a link.
(Other definitions for stench that I've seen before include "Terrible smell" , "Dreadful pong" , "Awful pong" , "Offensive odour" , "Stink" .)