Officer, one refusing to fight on his own, left (7)
I believe the answer is:
colonel
'officer' is the definition.
(colonel is a kind of officer)
'one refusing to fight on his own left' is the wordplay.
'one refusing to fight' becomes 'CO' (conscientious objector).
'on his own' becomes 'lone' (all alone).
'left' becomes 'l' (common abbreviation).
'co'+'lone'+'l'='COLONEL'
(Other definitions for colonel that I've seen before include "Senior army officer" , "Army three downn" , "Army officer ranking below a brigadier" , "Rank (below brigadier)" , "High-ranking officer" .)