The Collaborative International Dictionary of English v.0.48:

Clannish \Clan"nish\, a.
   Of or pertaining to a clan; closely united, like a clan;
   disposed to associate only with one's clan or clique;
   actuated by the traditions, prejudices, habits, etc., of a
   clan. -- Clan"nish*ly, adv. -- clan"nish*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

clannish
    adj 1: characteristic of a clan especially in being unified;
           "clannish loyalty"
    2: befitting or characteristic of those who incline to social
       exclusiveness and who rebuff the advances of people
       considered inferior [syn: clannish, cliquish, clubby,
       snobbish, snobby]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "clannish":
   arrogant, cliquey, cliquish, contemptuous, contumelious,
   disdainful, elect, ethnic, ethnocentric, exclusive, family,
   genetic, gentile, gentilic, haughty, lineal, national, phyletic,
   phylogenetic, racial, scornful, select, selective, sneering,
   sniffy, snobbish, snobby, snooty, snotty, supercilious,
   toploftical, toplofty, totemic, tribal, withering