Over the moon, having climbed Everest? (2,3,2,3,5)
I believe the answer is:
on top of the world
I believe this clue is a double definition.
'over the moon' is the first definition.
'having climbed everest?' is the second definition.
(Other definitions for on top of the world that I've seen before include "as Hillary once was?" , "Elated" , "Delighted" , "as a polar bear, but not as a penguin?" , "as a successful North Pole expedition?" .)