Presumably they don't take part in open arrest! (6,6)

I believe the answer is:

secret police

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'arrest' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

I don't understand the rest of the clue.

Can you help me to learn more?

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

I've seen this clue in The Irish News.

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