Bird in box-file? (7)
I believe the answer is:
sparrow
'bird' is the definition.
(sparrow is a kind of bird)
'box-file?' is the wordplay.
'box' becomes 'spar' (spar can mean to box).
'file?' becomes 'row' (a file is a row of squares e.g. on a chess board).
'spar'+'row'='SPARROW'
'in' acts as a link.
(Other definitions for sparrow that I've seen before include "Pirate" , "Small brownish bird" , "Finch-like bird" , "Singing bird" , "Bird with cockney associations" .)