Old jailer from country imprisoning knight (7)
I believe the answer is:
turnkey
'old jailer' is the definition.
(historical term for a jailer)
'country imprisoning knight' is the wordplay.
I cannot really see how this works, but
'knight' could be 'n' (chess abbreviation) and 'n' is found within the answer.
'country' could be 'turkey' (Turkey is an example) and 'turkey' is found within the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'from' is the link.
Can you help me to learn more?
(Other definitions for turnkey that I've seen before include "[SCREW]" , "ready to start working immediately" , "locker" , "Jailer (arch.)" , "Gaoler" .)