Awful smell from small fish (6)
I believe the answer is:
stench
'awful 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'
'from' is the link.
(Other definitions for stench that I've seen before include "Foul odour" , "A really foul smell" , "A really vile smell" , "Awful pong" , "A really bad smell" .)