One arrangement is perfect (5)
I believe the answer is:
ideal
'is perfect' is the definition.
(similar in meaning)
'one arrangement' is the wordplay.
'one' becomes 'I' (Roman numeral).
'arrangement' becomes 'deal' (arrangement between two people or groups).
'i'+'deal'='IDEAL'
(Other definitions for ideal that I've seen before include "(the best one can get)" , "fanciful" , "Paradigm" , "Dream" , "To which one aspires" .)