Stop said form of payment (6)
I believe the answer is:
cheque
'form of payment' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for cheque that I've seen before include "Bill of exchange" , "Signed money order" , "A payment order form" , "Written order directing a bank to pay money" , "Written authorisation to pay money from bank account" .)