Hand in substantial payment (8)
I believe the answer is:
remittal
'payment' is the definition.
(I know that remittal is a type of payment)
'hand in substantial' is the wordplay.
'hand' becomes 'mitt' (synonyms).
'in' is an insertion indicator.
'substantial' becomes 'real' (synonyms).
'mitt' put into 'real' is 'REMITTAL'.
(Other definitions for remittal that I've seen before include "action to cancel obligation?" , "reference" , "transfer" , "Handing over" .)