Box held in reserve, not closing (4)
I believe the answer is:
spar
'box' is the definition.
(spar can mean to box)
'reserve not closing' is the wordplay.
'reserve' becomes 'spare' (synonyms).
'not closing' means to remove the last letter.
'spare' with its final letter taken away is 'SPAR'.
'held in' is the link.
(Other definitions for spar that I've seen before include "Crystalline mineral" , "Box; mineral; pole" , "piece of wood?" , "Do up (bird for oven)" , "Box - girder" .)