Boast to a 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)
'bird' is the second definition.
(crow is a kind of bird)
'to a' acts as a link.
(Other definitions for crow that I've seen before include "Brag; carrion bird" , "Boast gloatingly, American Indian" , "Express gleeful triumph" , "Boast, swagger, triumph" , "Common bird" .)