Backing a writer or a flier (5)
I believe the answer is:
robin
'a flier' is the definition.
(robin is a kind of birdanimal)
'backing a writer or' is the wordplay.
'backing' says the letters should be written in reverse.
'a writer' becomes 'nib' (something which writes).
'nib'+'or'='nibor'
'nibor' reversed gives 'ROBIN'.
(Other definitions for robin that I've seen before include "Christmas songbird" , "Friendly winter bird" , "Batman and --" , "Garden bird" , "Batman's companion" .)