Keep it classified, but monitor those working to crush government's opponents (6,6)

I believe the answer is:

secret police

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'crush government's opponents' is the definition.
The answer and definition can be both groups as well as being singular nouns.
Perhaps you can see a link between them that I can't see?

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for secret police that I've seen before include "Undercover lawmen" , "NKVD, perhaps" , "Anti-subversion force" .)

I've seen this clue in The Irish Times.

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