Element of law enforcement? (6)

I believe the answer is:

copper

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'element' is the definition.
(copper is a kind of element)

I can't explain the rest of the clue.

(Other definitions for copper that I've seen before include "Coin, policeman or metallic element" , "bluebottle" , "A reddish-brown metal" , "Ductile metal" , "Metallic element and British bobby" .)

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

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