Guard's course of action finally changed (6)
I believe the answer is:
police
'guard's' is the definition.
(policing is a kind of guarding)
I don't understand the remainder of the clue.
(Other definitions for police that I've seen before include "Lawmen" , "9a 19d" , "Branch of the security forces" , "Heat in US" , "Law enforcers" .)