Secret police in almost unmoving position (5)

I believe the answer is:

stasi

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'secret police' is the definition.
(state security service of the GDR, East Germany)

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

Can you help me to learn more?

(Other definitions for stasi that I've seen before include "Secret police of the former German Democratic Republic" , "East German Secret Police" , "Former security force" , "Force behind Wall" , "old secret service" .)

I've seen this clue in The Guardian.

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