Allegation about mail dispute (5)
I believe the answer is:
claim
'allegation' is the definition.
(I've seen this before)
'about mail dispute' is the wordplay.
'about' becomes 'c' (abbreviation for circa).
'dispute' indicates anagramming the letters (I've seen 'disputed' mean this).
'mail' anagrammed gives 'laim'.
'c'+'laim'='CLAIM'
(Other definitions for claim that I've seen before include "Formal request" , "Call" , "State to be the case" , "Allegation" , "Assertion of right" .)