A service set up miles away (4)
I believe the answer is:
afar
'miles away' is the definition.
(far away)
'a service set up' is the wordplay.
'service' becomes 'RAF' (Royal Air Force).
'set up' shows that the letters should be reversed in order (in a down clue, letters go up).
'raf' in reverse letter order is 'far'.
'a'+'far'='AFAR'
(Other definitions for afar that I've seen before include "a good way off" , "Not in this neighbourhood" , "A long distance away" , "At a distance (poetic)" , "Many a mile" .)