Complain about bum rap (4)
I believe the answer is:
carp
'complain' is the definition.
(thesaurus)
'about bum rap' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa'**).
'bum' is an anagram indicator (bum can mean faulty or wrong).
'rap' is an anagram of 'arp'.
'c'+'arp'='CARP'
(Other definitions for carp that I've seen before include "Fish - complain" , "Go on at" , "Raise petty objections about goldfish" , "Complain continually" , "praise? The opposite" .)