Mostly arrest snoopers (5)

I believe the answer is:

stasi

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'snoopers' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'mostly arrest' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'arrest' becomes 'stasis' (I've seen this before).
'stasis' with its last letter removed is 'STASI'.

Can you help me to learn more?

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

I've seen this clue in The Guardian.

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