One transaction that's flawless (5)
I believe the answer is:
ideal
'flawless' is the definition.
(I've seen this in another clue)
'one transaction' is the wordplay.
'one' becomes 'i' (Roman numeral).
'transaction' becomes 'deal' (a deal is a transaction or exchange).
'i'+'deal'='IDEAL'
'that's' is the link.
(Other definitions for ideal that I've seen before include "Standard of perfection or excellence" , "highly desirable" , "Optimal" , "(the best one can get)" , "Model of ethical behaviour" .)