Singular fish smell (6)
I believe the answer is:
stench
'smell' is the definition.
(stench is a kind of smell)
'singular fish' is the wordplay.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'fish' becomes 'tench' (tench is a kind of fish).
's'+'tench'='STENCH'
(Other definitions for stench that I've seen before include "A really foul smell" , "Dreadful pong" , "Evidence of corruption" , "Horrible smell" , "Foul odour" .)