Rebel duke isn't holding party (9)
I believe the answer is:
dissident
'rebel?' is the definition.
(I know that rebel can be written as dissident)
'duke isn't holding party' is the wordplay.
'duke' becomes 'd' (abbreviation).
'holding' is an insertion indicator.
'party' becomes 'side' (eg a side or party in a contract).
'isnt' going around 'side' is 'issident'.
'd'+'issident'='DISSIDENT'
(Other definitions for dissident that I've seen before include "Person opposing official policy" , "Protester" , "Opposer of officialdom" , "One opposed to official policy" , "Rebel, protestor" .)