A military force retreating at a distance (4)
I believe the answer is:
afar
'at a distance' is the definition.
(I've seen this before)
'a military force retreating' is the wordplay.
'military force' becomes 'raf' (Royal Air Force**).
'retreating' says the letters should be written backwards.
'raf' reversed gives 'far'.
'a'+'far'='AFAR'
(Other definitions for afar that I've seen before include "Inhabitant of Ethiopia" , "(From/at) a great distance" , "Many miles away" , "Distant, not near" , "At a distance (poetic)" .)