Pilot first ascending by means of the mountain peak (7)
I believe the answer is:
aviator
'pilot' is the definition.
(I've seen this in another clue)
'first ascending by means of the mountain peak' is the wordplay.
'first ascending' becomes 'a' (1st letter of 'ascending').
'by means of' becomes 'via' (via can mean 'by means of' or 'using').
'the mountain peak' becomes 'tor' (I've seen this in another clue).
'a'+'via'+'tor'='AVIATOR'
(Other definitions for aviator that I've seen before include "Ace, say," , "He travels up" , "Flyer in early days of planes" , "Navigator of the air" , "Aeronaut" .)