It won't show to one's credit when one's in debt (5)
I believe the answer is:
debit
'it won't show to one's credit' is the definition.
The definition suggests a singular noun which matches the answer.
'one's in debt' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'i' placed inside 'debt' is 'DEBIT'.
'when' is the link.
Can you help me to learn more?
(Other definitions for debit that I've seen before include "Accounting entry showing sum owing" , "Negative entry in the accounts" , "Opposite of credit" , "Remove (money) from account" , "Accounting entry showing amount spent" .)