Company doctor at a fight (6)
I believe the answer is:
combat
'a fight' is the definition.
(I know that fight can be written as combat)
'company doctor at' is the wordplay.
'company' becomes 'co' (abbreviation).
'doctor' becomes 'MB' (Bachelor of Medicine).
'co'+'mb'+'at'='COMBAT'
(Other definitions for combat that I've seen before include "Military fighting" , "Fight, struggle or contest" , "Military action" , "Fight - strive against" , "sort of jacket" .)