Canadian policeman allowing second to go free (5)
I believe the answer is:
untie
'free' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
(Other definitions for untie that I've seen before include "Remove ropes, unfasten" , "Let loose" , "Set free" , "Free from restraining bonds or ropes" , "Loosen ropes or knots" .)