Pilot beginning to ascend using hill (7)
I believe the answer is:
aviator
'pilot' is the definition.
(I've seen this before)
'beginning to ascend using hill' is the wordplay.
'beginning to' suggests taking the first letters.
'using' becomes 'via' (I've seen this in another clue).
'hill' becomes 'tor' (tor is a kind of hill).
The first letter of 'ascend' is 'a'.
'a'+'via'+'tor'='AVIATOR'
(Other definitions for aviator that I've seen before include "Navigator of the air" , "Flyer, pilot" , "Pilot of plane" , "Aeronaut" , "Pilot; style of sunglasses" .)