Fish causing bellyache (4)
I believe the answer is:
carp
This clue is a double definition.
'fish' is the first definition.
(carp is a kind of fish)
'bellyache' is the second definition.
(I've seen this in another clue)
'causing' is the link.
(Other definitions for carp that I've seen before include "Raise trivial objections" , "Criticise - fish" , "Fish often kept in ponds" , "Keep complaining" , "Fish - complain" .)