Duke insisted reforming rebel (9)
I believe the answer is:
dissident
'rebel' is the definition.
(I've seen this before)
'duke insisted reforming' is the wordplay.
'duke' becomes 'd' (abbreviation).
'reforming' indicates an anagram.
'insisted' anagrammed gives 'issident'.
'd'+'issident'='DISSIDENT'
(Other definitions for dissident that I've seen before include "Rebel, protestor" , "Political nonconformist" , "Protester" , "Insets did for one who opposes authority" , "Opposer of official line" .)