One transaction that's above reproach (5)
I believe the answer is:
ideal
'above reproach' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'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.
Can you help me to learn more?
(Other definitions for ideal that I've seen before include "fanciful" , "Exemplary" , "Something that one hopes to attain" , "Coal-mining waste" , "Conceptual" .)