Box not required if base missing (4)
I believe the answer is:
spar
'box' is the definition.
(spar can mean to box)
'not required if base missing' is the wordplay.
'not required' becomes 'spare' (I've seen this before).
'if base missing' means to remove the last letter (in a down clue, the bottom letter is missing).
'spare' with its final letter taken away is 'SPAR'.
(Other definitions for spar that I've seen before include "supporting member" , "Box for practice" , "Box in training - mineral" , "Shadow box" , "Crystal - box" .)