The one bit of business that's best (5)
I believe the answer is:
ideal
'best' is the definition.
(similar in meaning)
'the one bit of business' is the wordplay.
'the one' becomes 'i' (Roman numeral).
'bit of business' becomes 'deal' (I've seen this in another clue).
'i'+'deal'='IDEAL'
'that's' acts as a link.
(Other definitions for ideal that I've seen before include "Just perfect, couldn't be better" , "An - - Husband (Wilde)" , "Utopian; yardstick" , "Standard to aim for" , "Perfect but not likely to become a reality" .)