One transaction is perfect (5)
I believe the answer is:
ideal
'is perfect' is the definition.
(similar in meaning)
'one transaction' is the wordplay.
'one' becomes 'i' (Roman numeral).
'transaction' becomes 'deal' (a deal is a transaction or exchange).
'i'+'deal'='IDEAL'
(Other definitions for ideal that I've seen before include "Perfect but perhaps unlikely to become reality" , "The best conceivable" , "just the job" , "existing only in imagination" , "supreme" .)