Not the only thing a poor painter cannot draw (6)
I believe the answer is:
cheque
'draw' is the definition.
(I know that cheque is a more specific form of the action draw)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for cheque that I've seen before include "A payment order form" , "Personal bank order" , "Method of payment" , "Bank draft" , "Bank document" .)