Small fish producing appalling smell (6)
I believe the answer is:
stench
'appalling smell' is the definition.
(a stench is a terrible smell)
'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 "Horrid smell" , "Miasma" , "A really bad smell" , "A really vile smell" , "Reek" .)