Doctor let loose officer on horseback (7)

I believe the answer is:

mountie

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'officer on horseback' is the definition.
'Mountie' can be an answer for 'officer' (Mountie is an example). I am not sure about the 'on horseback' bit.

'doctor let loose' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'let loose' becomes 'untie' (I've seen this in another clue).
'mo'+'untie'='MOUNTIE'

Can you help me to learn more?

(Other definitions for mountie that I've seen before include "Canadian police officer" , "Member of the Royal Canadian Mounted Police" , "Horse-riding Canadian cop" , "Canadian policeman" , "One policeman" .)

I've seen this clue in the Sydney Morning Herald.

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