Legally required to stop imprisoning teacher (9)
I believe the answer is:
statutory
'legally required' is the definition.
(as in statutory rights)
'stop imprisoning teacher' is the wordplay.
'stop' becomes 'stay' (stay is a kind of stop).
'imprisoning' is an insertion indicator.
'teacher' becomes 'tutor' (tutor is a kind of teacher).
'stay' enclosing 'tutor' is 'STATUTORY'.
'to' acts as a link.
(Other definitions for statutory that I've seen before include "Legal" , "Created by law" , "Prescribed or authorised by law" , "Fixed or required by law" , "Enshrined in law" .)