Disciplinarian's right to stop unruly inmate (8)
I believe the answer is:
martinet
'disciplinarian's' is the definition.
(synonyms)
'right to stop unruly inmate' is the wordplay.
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'to stop' means one lot of letters goes inside another (stopping can mean blocking a gap).
'unruly' indicates anagramming the letters.
'inmate' with letters rearranged gives 'mainet'.
'rt' going within 'mainet' is 'MARTINET'.
(Other definitions for martinet that I've seen before include "A rigorous operator" , "Person who demands strict obedience" , "drillmaster" , "One demands exact conformity to rules" , "A strict disciplarian" .)