Encounter a doctor in a coat (6)
I believe the answer is:
combat
'encounter' is the definition.
(thesaurus)
'a doctor in a coat' is the wordplay.
'a doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'in' means one lot of letters goes inside another.
'mb' put within 'coat' is 'COMBAT'.
(Other definitions for combat that I've seen before include "Oppose" , "Battle, warfare" , "Military action" , "Fight, struggle or contest" , "Conflict" .)