Abuse from security service, apparently a special experience (8)

I believe the answer is:

mistreat

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'abuse' is the definition.
(I know that abuse can be written as mistreat)

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

(Other definitions for mistreat that I've seen before include "Behave badly towards" , "Handle badly" , "Abuse" , "Handle unfairly" , "Ill-use" .)

I've seen this clue in The Guardian.

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