Tolerate a big-headed flier (5)
I believe the answer is:
brook
'tolerate' is the definition.
(synonyms)
'a big-headed flier' is the wordplay.
'a big-headed' becomes 'b' (this might be a standard abbreviation which I don't know about).
'flier' becomes 'rook' (rook is a kind of birdanimal).
'b'+'rook'='BROOK'
Can you help me to learn more?
(Other definitions for brook that I've seen before include "Stream; put up with" , "running water" , "Stand for" , "stomach" , "Allow" .)