He needs to settle (6)
I believe the answer is:
debtor
'he' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'to settle' is the wordplay.
I cannot really understand how this works, but
'to' is within the answer.
This may be the basis of the clue (or it may be nonsense).
'needs' acts as a link.
Can you help me to learn more?
(Other definitions for debtor that I've seen before include "Person owing money" , "Person who owes money" , "Opposite of creditor" , "One of The Borrowers" , "Someone owing money" .)