Bill isn't paid by him (6)
I believe the answer is:
debtor
'bill isn't paid by him' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is the entire clue.
Can you help me to learn more?
(Other definitions for debtor that I've seen before include "One of The Borrowers" , "Opposite of creditor" , "Borrower" , "One who owes money" , "Anyone owing money" .)