WordNet (r) 3.0 (2006):

agent 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:

28 Moby Thesaurus words for "agent provocateur":
   agitator, agitprop, capper, catalyst, come-on man, decoy,
   demagogue, exciter, firebrand, fomenter, incendiary, inciter,
   inflamer, instigator, mischief-maker, plant, provocateur, provoker,
   rabble-rouser, ringleader, rouser, seditionary, seditionist, shill,
   stool pigeon, stoolie, troublemaker, urger