Pilot starts ascent, travelling through a rocky height (7)
I believe the answer is:
aviator
'pilot' is the definition.
(I've seen this before)
'starts ascent travelling through a rocky height' is the wordplay.
'starts' indicates taking the first letters.
'travelling through' becomes 'via' (I've seen this in other clues).
'a rocky height' becomes 'tor' (I've seen this in another clue).
The initial letter of 'ascent' is 'a'.
'a'+'via'+'tor'='AVIATOR'
(Other definitions for aviator that I've seen before include "Airman" , "Aeronaut" , "Flier" , "One flying aircraft" , "one in a crate?" .)