Rebellious Democrat insisted on reform (9)
I believe the answer is:
dissident
'rebellious' is the definition.
(dissident means rebellious or mutinous)
'democrat insisted on reform' is the wordplay.
'democrat' becomes 'd'.
'on' says to put letters next to each other.
'reform' is an anagram indicator (letters in a new form).
'd' after 'insisted' is 'insistedd'.
'insistedd' anagrammed gives 'DISSIDENT'.
(Other definitions for dissident that I've seen before include "Political nonconformist" , "Rebel, protestor" , "Opposer of officialdom" , "One opposed to official policy" , "Opposer of official line" .)