Tail of poisonous fish produces awful smell (6)
I believe the answer is:
stench
'awful smell' is the definition.
(I've seen this before)
'tail of poisonous fish' is the wordplay.
'tail of' says to take the final letters.
'fish' becomes 'tench' (tench is a kind of fish).
The last letter of 'poisonous' is 's'.
's'+'tench'='STENCH'
'produces' is the link.
(Other definitions for stench that I've seen before include "Awful pong" , "A really vile smell" , "Miasma" , "Foul odour" , "Strong unpleasant smell" .)