Stop being a policeman? (6)
I believe the answer is:
arrest
'stop' is the definition.
(arrest is a kind of stop)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for arrest that I've seen before include "Collar" , "Seize and take into custody" , "Cause to stop" , "Seize (a crime suspect)" , "Stop - detain" .)