Missouri frees Canadian police (8)
I believe the answer is:
mounties
'canadian police' is the definition.
(nickname for the Royal Canadian Mounted Police)
'missouri frees' is the wordplay.
'missouri' becomes 'mo' (abbreviation for the US state).
'frees' becomes 'unties' (I've seen this in another clue).
'mo'+'unties'='MOUNTIES'
(Other definitions for mounties that I've seen before include "Police force of Canada" , "Canadian federal police" .)