Bird box with chain (7)
I believe the answer is:
sparrow
'bird' is the definition.
(sparrow is a kind of bird)
'box with chain' is the wordplay.
'box' becomes 'spar' (spar can mean to box).
'with' says to put letters next to each other.
'chain' becomes 'row'.
'spar'+'row'='SPARROW'
(Other definitions for sparrow that I've seen before include "Little brown bird" , "one on the wing" , "Cock Robin's killer" , "Small brown bird" , "Winger" .)