Box mostly surplus to requirements (4)
I believe the answer is:
spar
'box' is the definition.
(spar can mean to box)
'mostly surplus to requirements' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'surplus to requirements' becomes 'spare' (I've seen this before).
'spare' with its last letter removed is 'SPAR'.
(Other definitions for spar that I've seen before include "Yard" , "Wooden pole -- box" , "Box for practice" , "Practise boxing with a pole?" , "supporting member" .)