Pilots advanced westwards at a great distance (4)
I believe the answer is:
afar
'at a great distance' is the definition.
(I've seen this before)
'pilots advanced westwards' is the wordplay.
'pilots' becomes 'RAF' (Royal Air Force).
'advanced' becomes 'a'.
'westwards' says the letters should be written backwards (letters go right to left or from 'east' to 'west').
'raf'+'a'='rafa'
'rafa' written backwards gives 'AFAR'.
(Other definitions for afar that I've seen before include "At a distance (poetic)" , "A long distance away" , "A long way away" , "Inhabitant of Ethiopia" , "beyond the horizon" .)