Returned box containing special files (5)
I believe the answer is:
rasps
'files' is the definition.
(rasp is a kind of file)
'returned box containing special' is the wordplay.
'returned' shows that the letters should be reversed in order.
'box' becomes 'spar' (sparring is a kind of boxing).
'containing' means one lot of letters goes inside another.
'special' becomes 'S' (common abbreviation as in Special Air Service).
'spar' placed around 's' is 'spsar'.
'spsar' reversed gives 'RASPS'.
(Other definitions for rasps that I've seen before include "Grates harshly" , "Scrapes" , "Coarse files" , "Jars" , "fruit briefly" .)