Pilot beginning to advance by way of hill (7)
I believe the answer is:
aviator
'pilot' is the definition.
(I've seen this in another clue)
'beginning to advance by way of hill' is the wordplay.
'beginning to' suggests taking the first letters.
'by way of' becomes 'via' (via means by way of or by means of).
'hill' becomes 'tor' (tor is a kind of hill).
The initial letter of 'advance' is 'a'.
'a'+'via'+'tor'='AVIATOR'
(Other definitions for aviator that I've seen before include "Aircraft pilot" , "Aeronaut" , "Navigator of the air" , "Airman" , "Air pilot" .)