The Collaborative International Dictionary of English v.0.48:

Ringleader \Ring"lead`er\, n.
   1. The leader of a circle of dancers; hence, the leader of a
      number of persons acting together; the leader of a herd of
      animals.
      [1913 Webster]

            A primacy of order, such an one as the ringleader
            hath in a dance.                      --Barrow.
      [1913 Webster]

   2. Opprobriously, a leader of a body of men engaged in the
      violation of law or in an illegal enterprise, as rioters,
      mutineers, or the like.
      [1913 Webster]

            The ringleaders were apprehended, tried, fined, and
            imprisoned.                           --Macaulay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ringleader
    n 1: a person who leads (especially in illicit activities)

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "ringleader":
   Mahdi, agent provocateur, agitator, agitprop, bell cow, bell mare,
   bellwether, born leader, boss, cacique, catalyst,
   charismatic leader, choirmaster, choragus, conductor, coryphaeus,
   demagogue, duce, exciter, file leader, firebrand, fomenter,
   forerunner, fugleman, incendiary, inciter, inflamer,
   inspired leader, instigator, keynoter, leader, leader of men,
   man higher up, messiah, mischief-maker, pacemaker, pacesetter,
   policy maker, precentor, provocateur, provoker, rabble-rouser,
   rouser, sachem, seditionary, seditionist, standard-bearer,
   symphonic conductor, torchbearer, troublemaker, urger