Men helping to defend box and scoring (13)
I believe the answer is:
orchestration
'scoring' is the definition.
(I've seen this before)
'men helping to defend box' is the wordplay.
'men' becomes 'OR' (Other Ranks: non-commissioned soldiers).
'helping' becomes 'ration' (a ration is a serving or helping).
'to defend' is an insertion indicator.
'box' becomes 'chest' (chest is a kind of box).
'or'+'ration'='orration'
'orration' going around 'chest' is 'ORCHESTRATION'.
'and' acts as a link.
(Other definitions for orchestration that I've seen before include "Musical arrangement" , "Note arrangement" , "Scoring for strings, brass, woodwind and percussion" , "Arrangement for instruments - short reaction (anag)" , "Stage-management" .)