Fomenter, prime mover (10)
I believe the answer is:
I believe this is a double definition.
'fomenter' is the first definition.
'prime mover' is the second definition.
(I've seen this before)
(Other definitions for instigator that I've seen before include "He incites" , "One setting off" , "starter" , "One who initiates (something)" , "Stirrer" .)