Boast to a flyer (4)
I believe the answer is:
crow
'boast' is the definition.
(to crow about something is to boast about it)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for crow that I've seen before include "Brave" , "Black bird with raucous call" , "Native American -- bird" , "Express gleeful triumph" , "Common bird" .)