Best conceivable: one business arrangement (5)
I believe the answer is:
ideal
'best conceivable' is the definition.
(the best that can be imagined)
'one business arrangement' is the wordplay.
'one' becomes 'i' (Roman numeral).
'business arrangement' becomes 'deal' (eg a deal between two businesses).
'i'+'deal'='IDEAL'
(Other definitions for ideal that I've seen before include "Model of ethical behaviour" , "Paragon" , "Perfectly suited" , "Benchmark" , "To which one aspires" .)