Boast about rank (4)
I believe the answer is:
crow
'boast' is the definition.
(to crow about something is to boast about it)
'about rank' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'rank' becomes 'row' (I've seen this in another clue).
'c'+'row'='CROW'
(Other definitions for crow that I've seen before include "Black bird; brag" , "One joins others in a murder" , "Ugly old woman" , "Large black bird" , "Brag; carrion bird" .)