One transaction is perfect (5)
I believe the answer is:
Here is my best explanation:
'is perfect' is the definition.
('ideal' can be similar in meaning to 'perfect')
'one transaction' is the subsidiary indication.
'one' becomes 'i' (roman numeral).
'transaction' becomes 'deal' (deal is a kind of transaction).
(Other definitions for ideal that I've seen before include "Standard to aim for", "Utopian; yardstick", "A deli (anag.)", "Perfect but perhaps unlikely to become reality", "Model of ethical behaviour", "Perfect for the purpose", "Just the ticket", "Ultimate standard of perfection", "Model of perfection" and "Paragon".)