Hundred detained by correct law (5)
I believe the answer is:
edict
'law' is the definition.
(thesaurus)
'hundred detained by correct' is the wordplay.
'hundred' becomes 'c' (Roman numeral).
'detained by' means one lot of letters goes inside another (I've seen this in other clues).
'correct' becomes 'edit' (I've seen this before**).
'c' going inside 'edit' is 'EDICT'.
(Other definitions for edict that I've seen before include "Authoritative ruling" , "An official command" , "Declaration" , "Legally-binding order" , "Canon" .)