It's no credit when one's in debt (5)
I believe the answer is:
debit
'it's no credit' is the definition.
I can't tell whether this defines the answer.
'one's in debt' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'i' placed into 'debt' is 'DEBIT'.
'when' acts as a link.
Can you help me to learn more?
(Other definitions for debit that I've seen before include "Opposite of credit" , "Money owed" , "Deduct from account" , "Sum taken from account" , "Negative entry in the accounts" .)