Troublemaker (8)
'troublemaker' is the definition.
(agitator is a kind of troublemaker)
This is all the clue.
(Other definitions for agitator that I've seen before include "One stirring up (things)" , "Stirrer up of public feeling" , "Provocateur or rabble-rouser" , "Disturber of the peace" , "One whipping up concern" .)
(Other definitions for rebel that I've seen before include "Mutineer, one who defies authority" , "He resists authority" , "Insurgent, renegade" , "Non-conformist" , "Rise up - dissent" .)
'troublemaker' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for stirrer that I've seen before include "Implement for mixing" , "One who spreads frightening or malicious rumours" , "engineering several revolutions in Darjeeling?" , "Scaremonger" , "Troublemaker (informal)" .)