Backing writer or flier (5)
I believe the answer is:
robin
'flier' is the definition.
(robin is a kind of birdanimal)
'backing writer or' is the wordplay.
'backing' is a reversal indicator.
'writer' becomes 'nib' (something which writes).
'nib'+'or'='nibor'
'nibor' back-to-front is 'ROBIN'.
(Other definitions for robin that I've seen before include "boy" , "Small songbird" , "Batman's assistant" , "Popular British bird" , "Bird on many cards" .)