Perfect one to buy and sell (5)
I believe the answer is:
ideal
'perfect' is the definition.
(similar in meaning)
'one to buy and sell' is the wordplay.
'one' becomes 'i' (Roman numeral).
'to buy and sell' becomes 'deal' (I've seen this before).
'i'+'deal'='IDEAL'
(Other definitions for ideal that I've seen before include "faultless" , "Optimum" , "Perfect; visionary" , "Perfect but not likely to become a reality" , "Something that one hopes to attain" .)