Small fish stink (6)
I believe the answer is:
stench
'stink' is the definition.
('stench' can be a synonym of 'stink')
'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'
(Other definitions for stench that I've seen before include "Evidence of corruption" , "Hum" , "A really bad smell" , "Appalling smell" , "Reek" .)