One agreement is perfect (5)
I believe the answer is:
ideal
'is perfect' is the definition.
(similar in meaning)
'one agreement' is the wordplay.
'one' becomes 'i' (Roman numeral).
'agreement' becomes 'deal' (deal is a kind of agreement).
'i'+'deal'='IDEAL'
(Other definitions for ideal that I've seen before include "Something that one hopes to attain" , "Exemplary" , "immaculate" , "supreme" , "Paradigm" .)