One transaction that can't be better (5)
I believe the answer is:
ideal
'one transaction that can't be better' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I can't see?
'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 "fanciful" , "Exemplar" , "faultless" , "Perfect but not likely to become a reality" , "without a flaw" .)