Policeman and army doctor set free (7)
I believe the answer is:
mountie
'policeman' is the definition.
(Royal Canadian Mounted Police officer)
'army doctor set free' is the wordplay.
'army doctor' becomes 'mo' (medical officer).
'set free' becomes 'untie' (I've seen this before).
'mo'+'untie'='MOUNTIE'
'and' acts as a link.
(Other definitions for mountie that I've seen before include "Member of the Royal Canadian Mounted Police" , "Canadian lawman" , "Horse-riding Canadian cop" , "The lawman always gets his man!" , "Canadian cop on a horse?" .)