Box file for a flier (7)
I believe the answer is:
sparrow
'a flier' is the definition.
(sparrow is a kind of birdanimal)
'box file' is the wordplay.
'box' becomes 'spar' (sparring is a kind of boxing).
'file' becomes 'row' (a file is a row of squares e.g. on a chess board).
'spar'+'row'='SPARROW'
'for' is the link.
(Other definitions for sparrow that I've seen before include "a British resident" , "Singing bird" , "Winger" , "Passerine bird" , "A small chirpy city bird 97)" .)