WordNet (r) 3.0 (2006):

duce
    n 1: leader; "Mussolini was called Il Duce"

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "duce":
   Mahdi, Simon Legree, absolute monarch, absolute ruler,
   all-powerful ruler, arrogator, autarch, autocrat, bell cow,
   bell mare, bellwether, born leader, caesar, charismatic leader,
   choirmaster, choragus, commissar, conductor, coryphaeus, czar,
   despot, dictator, disciplinarian, driver, file leader, forerunner,
   fugleman, hard master, inspired leader, leader, leader of men,
   martinet, messiah, oligarch, oppressor, pacemaker, pacesetter,
   pharaoh, precentor, ringleader, slave driver, standard-bearer,
   stickler, strong man, symphonic conductor, torchbearer, tyrant,
   usurper, warlord