Not quite certain matter involves large balance (7)
I believe the answer is:
surplus
'balance' is the definition.
(I've seen this before)
'not quite certain matter involves large' is the wordplay.
'not quite' means to remove the last letter.
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'matter' becomes 'pus' (pus is a kind of matter).
'involves' means one lot of letters goes inside another.
'large' becomes 'l'.
'sure' with its final letter taken away is 'sur'.
'pus' going around 'l' is 'plus'.
'sur'+'plus'='SURPLUS'
(Other definitions for surplus that I've seen before include "Excess, extra" , "a bit over the top" , "Superabundance" , "Remainder" , "balance" .)