A nobody (who thinks he's somebody) (7)
I believe the answer is:
upstart
(Other definitions for upstart that I've seen before include "One who has risen suddenly to high position, and is arrogant with it" , "Person risen to high position, arrogant with it" , "someone pretentious" , "Bumptious parvenu" , "Person who behaves arrogantly in the new position" .)