Croatian boxers reportedly bringing carp (5,5)
I believe the answer is:
split hairs
'carp' is the definition.
(both can mean to complain or find fault)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for split hairs that I've seen before include "Worry about minutiae" , "Make pedantic distinctions" , "Quibble" , "cavil exceedingly" , "salon complaint?" .)