In the best of all possible worlds (7)
I believe the answer is:
ideally
'in the best of all possible worlds' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ideally that I've seen before include "for best results" , "in theory" , "in optimal way" , "Perfectly; most suitably" , "Lay idle (anag)" .)