Jailer's detention centre in the country (7)
I believe the answer is:
turnkey
'jailer's' is the definition.
(synonyms)
I can't explain the rest of the clue.
Can you help me to learn more?
(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" .)