Rebel is holding note demanding immediate attention (9)
I believe the answer is:
insurgent
'rebel' is the definition.
(synonyms)
'is holding note demanding immediate attention' is the wordplay.
'holding' indicates putting letters inside.
'note' becomes 'n' (abbreviation).
'demanding immediate attention' becomes 'urgent' (I've seen this before).
'is' placed around 'n' is 'ins'.
'ins'+'urgent'='INSURGENT'
(Other definitions for insurgent that I've seen before include "rising" , "Unresting like a rebel" , "freedom fighter" , "Rebel, revolutionary" , "Revolting" .)