Box left over is incomplete (4)
I believe the answer is:
spar
'box' is the definition.
(sparring is a kind of boxing)
'left over is incomplete' is the wordplay.
'left over' becomes 'spare' (I've seen this before).
'is incomplete' means to remove the last letter.
'spare' with its last letter removed is 'SPAR'.
(Other definitions for spar that I've seen before include "Box; various minerals" , "Do up (bird for oven)" , "bar" , "Practice boxing with wooden beam" , "Go a few rounds with the ship's pole" .)