Fellow doctor at fight (6)
I believe the answer is:
'fight' is the definition.
(I know that fight can be written as combat)
'fellow doctor at' is the wordplay.
'fellow' becomes 'co' (eg a co-worker is a fellow worker).
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
(Other definitions for combat that I've seen before include "Military fighting" , "A fight or contest, sometimes mortal" , "War; contest" , "Take action to deal with - fight" , "Oppose" .)