"In a perfect world . . ." (7)
I believe the answer is:
ideally
'in a perfect world' is the definition.
(in an ideal world)
This is all the clue.
(Other definitions for ideally that I've seen before include "In the best possible way or circumstances" , "In the best of all possible worlds" , "for preference" , "In the most perfect way" , "In perfect circumstances" .)