Most serious Sherpas start on mountain (8)
I believe the answer is:
severest
'most serious' is the definition.
'sherpas start on mountain' is the wordplay.
'start on' suggests taking the first letters (I've seen 'starts on' mean this).
'mountain' becomes 'everest' (Mount Everest).
The first letter of 'sherpas' is 's'.
's'+'everest'='SEVEREST'
(Other definitions for severest that I've seen before include "Most strict and harsh" , "Most extreme or difficult" , "Most harsh and punitive" , "Most dangerous" .)