Heading back, service area miles away (4)
I believe the answer is:
afar
'miles away' is the definition.
(far away)
'heading back service area' is the wordplay.
'heading back' says the letters should be written in reverse.
'service' becomes 'RAF' (Royal Air Force).
'area' becomes 'a' (maths abbreviation).
'raf'+'a'='rafa'
'rafa' in reverse letter order is 'AFAR'.
(Other definitions for afar that I've seen before include "beyond the horizon" , "quite some distance" , "Distant, not near" , "a good way off" , "Inhabitant of Ethiopia" .)