Make trifling complaints about the fish (4)
I believe the answer is:
carp
This clue might be a double definition.
'make trifling complaints' is the first definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'the fish' is the second definition.
(carp is a kind of fish)
'about' is the link.
Can you help me to learn more?
(Other definitions for carp that I've seen before include "Complain like goldfish" , "Raise petty objections about goldfish" , "Fish; nag pettily" , "Criticise - fish" , "Fish < and beef" .)