Verify said form of payment (6)
I believe the answer is:
cheque
'form of payment' is the definition.
(I've seen this before)
'verify said' is the wordplay.
'verify' becomes 'check' (checking is a kind of verifying).
'said' indicates a 'sounds like' (homophone) clue.
'check' sounds like 'CHEQUE'.
(Other definitions for cheque that I've seen before include "Bank draft" , "Paper payment" , "Bill of exchange" , "Bank document" , "Signed money order" .)