One flies aircraft primarily by way of rocky peak (7)
I believe the answer is:
aviator
'one flies' is the definition.
(an aviator flies planes)
'aircraft primarily by way of rocky peak' is the wordplay.
'primarily' suggests taking the first letters.
'by way of' becomes 'via' (via means by way of or by means of).
'rocky peak' becomes 'tor' (a tor is a rocky hill).
The initial letter of 'aircraft' is 'a'.
'a'+'via'+'tor'='AVIATOR'
(Other definitions for aviator that I've seen before include "Navigator of the air" , "one in a crate?" , "Pilot of plane" , "Airman" , "Aircraft pilot" .)