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).
'in' indicates putting letters inside.
'mb' going into 'coat' is 'COMBAT'.
(Other definitions for combat that I've seen before include "Oppose" , "Battle, warfare" , "Military action" , "Fight, struggle or contest" , "Conflict" .)