Before leaving Everest? (3)
I believe the answer is:
ere
'before' is the definition.
(poetic term)
'leaving everest?' is the wordplay.
'leaving' says the answer is hidden in the clue (the answer must leave the confines of the clue).
'ERE' is hidden within '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)" .)