Jailer's detention centre in the country (7)

I believe the answer is:

turnkey

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'jailer's' is the definition.
(synonyms)

I don't understand the rest of the clue.

(Other definitions for turnkey that I've seen before include "[SCREW]" , "Jailer (arch.)" , "Old guard" , "Designed and ready for immediate use by the purchaser" , "Jailer (archaic), (of service) provided ready for use" .)

I've seen this clue in the Evening Standard.

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