Songbird in series on box (7)
I believe the answer is:
sparrow
'songbird' is the definition.
(I've seen this before)
'series on box' is the wordplay.
'series' becomes 'row' (I've seen this before**).
'on' says to put letters next to each other.
'box' becomes 'spar' (sparring is a kind of boxing).
'row' after 'spar' is 'SPARROW'.
'in' acts as a link.
(Other definitions for sparrow that I've seen before include "House bird" , "Bird with cockney associations" , "a British resident" , "Small songbird" , "Little brown bird" .)