One opposing official policy -- leader of Democrats insisted upon change (9)
I believe the answer is:
dissident
'one opposing official policy' is the definition.
(I've seen this before)
'leader of democrats insisted upon change' is the wordplay.
'leader of' suggests taking the first letters.
'upon' says to put letters next to each other.
'change' indicates an anagram.
The first letter of 'democrats' is 'd'.
'd' after 'insisted' is 'insistedd'.
'insistedd' anagrammed gives 'DISSIDENT'.
(Other definitions for dissident that I've seen before include "One opposing official policy" , "Protester" , "Opposer of official line" , "Insets did for one who opposes authority" , "Opposer of officialdom" .)