One transaction that can't be faulted (5)
I believe the answer is:
ideal
'one transaction that can't be faulted' is the definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?
'one transaction' is the wordplay.
'one' becomes 'i' (Roman numeral).
'transaction' becomes 'deal' (a deal is a transaction or exchange).
'i'+'deal'='IDEAL'
Can you help me to learn more?
(Other definitions for ideal that I've seen before include "Just perfect, couldn't be better" , "existing only in imagination" , "Perfect for the purpose" , "Utopian; yardstick" , "Consummate" .)