One agreement that is perfect (5)
I believe the answer is:
ideal
'is perfect' is the definition.
('ideal' can be similar in meaning to 'perfect')
'one agreement' is the wordplay.
'one' becomes 'i' (Roman numeral).
'agreement' becomes 'deal' (deal is a kind of agreement).
'i'+'deal'='IDEAL'
'that' acts as a link.
(Other definitions for ideal that I've seen before include "Perfect but not likely to become a reality" , "Nonpareil" , "Dream" , "Paradigm" , "supreme" .)