Trouble-maker (9)
There are a few possibilities for this:
firebrand
(Other definitions for upstart that I've seen before include "One who has risen suddenly to high position, and is arrogant with it" , "Nouveau riche" , "Bold newcomer" , "Bumptious parvenu" , "Person recently elevated and arrogant with it" .)
(Other definitions for stirrer that I've seen before include "Implement for mixing" , "Troublesome" , "One deliberately causing trouble" , "One who spreads frightening or malicious rumours" , "One making solution perhaps" .)
(Other definitions for firebrand that I've seen before include "Agitator" , "Rabble-rouser" , "Energetic person often fomenting trouble" , "One who stirs up trouble" , "Troublemaker" .)