One writing a cheque for a back payment (6)
I believe the answer is:
drawer
'one writing a cheque' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a back payment' is the wordplay.
'a back' is a reversal indicator.
'payment' becomes 'reward' (reward is a kind of payment).
'reward' backwards is 'DRAWER'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for drawer that I've seen before include "One sliding into e.g. chest" , "Cheque signatory" , "One signing cheque" , "neither winner nor loser?" , "that's in a chest" .)