Complain unreasonably (4)
I believe the answer is:
carp
'complain unreasonably' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for carp that I've seen before include "Fish; find fault continually" , "Fish - grouse" , "Continually nag; fish" , "Go on at" , "Complain continually" .)