Soldier collects second acceptable sum (6)
I believe the answer is:
amount
'sum' is the definition.
('amount' can be a synonym of 'sum')
'soldier collects second acceptable' is the wordplay.
'soldier' becomes 'ant' (I've seen this before).
'collects' indicates putting letters inside.
'second' becomes 'mo' (both are short periods of time).
'acceptable' becomes 'u' ((Film classification)).
'mo'+'u'='mou'
'ant' enclosing 'mou' is 'AMOUNT'.
(Other definitions for amount that I've seen before include "Total or extent" , "Add up or be equivalent to" , "A quantity or total of anything" , "Size" , "Sum of money" .)