Doctor dons jacket for action (6)
I believe the answer is:
combat
'action' is the definition.
(combat is a kind of action)
'doctor dons jacket' is the wordplay.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'dons' is an insertion indicator (don means to wear or put on).
'jacket' becomes 'coat' (I've seen this before).
'mb' going into 'coat' is 'COMBAT'.
'for' acts as a link.
(Other definitions for combat that I've seen before include "Oppose" , "Fight, struggle or contest" , "type of jacket" , "Fight or contest, may be mortal" , "Armed fighting" .)