Boast about argument (4)
I believe the answer is:
crow
'boast' is the definition.
(to crow about something is to boast about it)
'about argument' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'argument' becomes 'row' (to row is to argue).
'c'+'row'='CROW'
(Other definitions for crow that I've seen before include "Bird - birdcall" , "Black bird; brag" , "Blow one's own trumpet" , "Boast, swagger, triumph" , "Boast gloatingly, American Indian" .)