Sherpa's first to leave most demanding mountain (7)
I believe the answer is:
everest
'mountain' is the definition.
(Mount Everest)
'sherpa's first to leave most demanding' is the wordplay.
'sherpa's first' becomes 's' (1st letter of 'sherpa').
'to leave' is a deletion indicator.
'most demanding' becomes 'severest' (severe can mean hard or demanding).
'severest' with 's' taken out is 'EVEREST'.
(Other definitions for everest that I've seen before include "Tee revs up the mountain" , "Mount, climbed 1953" , "Himalayan peak" , "One conquered in 1953" , "Himalayan mount" .)