WordNet (r) 3.0 (2006):

provocateur
    n 1: a secret agent who incites suspected persons to commit
         illegal acts [syn: agent provocateur, provocateur]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "provocateur":
   agent provocateur, agitator, agitprop, catalyst, demagogue,
   exciter, firebrand, fomenter, incendiary, inciter, inflamer,
   instigator, mischief-maker, provoker, rabble-rouser, ringleader,
   rouser, seditionary, seditionist, troublemaker, urger