Complains fish start to smell (5)
I believe the answer is:
carps
'complains' is the definition.
(I've seen this in another clue)
'fish start to smell' is the wordplay.
'fish' becomes 'carp' (carp is a kind of fish).
'start to' says to take the initial letters.
The initial letter of 'smell' is 's'.
'carp'+'s'='CARPS'
(Other definitions for carps that I've seen before include "Finds fault with" , "Makes trivial objections" .)