Before leaving Everest? (3)
I believe the answer is:
ere
'before' is the definition.
(usually literary)
'leaving everest?' is the wordplay.
'leaving' indicates a hidden word (the answer must leave the confines of the clue).
'ERE' can be found hidden inside 'everest'.
(Other definitions for ere that I've seen before include "Old prior" , "Before, poetically" , "Archaic word for before - a palindrome" , "Before (in poetry)" , "Before (in time)" .)