Boast; black bird (4)
I believe the answer is:
crow
I believe this is a double definition.
'boast' is the first definition.
(to crow about something is to boast about it)
'black bird' is the second definition.
(I've seen this in another clue)
(Other definitions for crow that I've seen before include "Make sound of cock - bird" , "Make sounds of pleasure as a baby does - or a bird" , "Black bird with raucous call" , "Boast gloatingly, American Indian" , "Black bird; boast" .)