Complain at fish (4)
I believe the answer is:
carp
I believe this is a double definition.
'complain' is the first definition.
(carp can informally mean to complain or whinge)
'fish' is the second definition.
(I know that carp is a type of fish)
'at' is the link.
(Other definitions for carp that I've seen before include "Swimmer" , "Keep complaining" , "Fish < and beef" , "Raise petty objections about goldfish" , "Raise trivial objections" .)