Extra short box (4)
I believe the answer is:
spar
'box' is the definition.
(spar can mean to box)
'extra short' is the wordplay.
'extra' becomes 'spare' (both can mean left over).
'short' means to remove the last letter.
'spare' with its last letter taken away is 'SPAR'.
(Other definitions for spar that I've seen before include "What boxers do" , "quarrel" , "Raps with large beam" , "Mineral; box; wing beam" , "Length of wood" .)