One portion is perfect (5)
I believe the answer is:
ideal
'is perfect' is the definition.
('ideal' can be similar in meaning to 'perfect')
'one portion' is the wordplay.
'one' becomes 'i' (Roman numeral).
'portion' becomes 'deal' (dealing is a kind of portioning).
'i'+'deal'='IDEAL'
(Other definitions for ideal that I've seen before include "without a flaw" , "Something that one hopes to attain" , "Model of ethical behaviour" , "Standard of perfection or excellence" , "Dream" .)