Smell first of some fish (6)
I believe the answer is:
stench
'smell' is the definition.
(stench is a kind of smell)
'first of some fish' is the wordplay.
'first of' indicates taking the first letters.
'fish' becomes 'tench' (tench is a kind of fish).
The first letter of 'some' is 's'.
's'+'tench'='STENCH'
(Other definitions for stench that I've seen before include "Appalling smell" , "Dreadful pong" , "smell, to put it mildly" , "Offensive odour" , "Awful pong" .)