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):

clannishly
    adv 1: in a clannish manner; "these four friends always act
           clannishly, and don't let us participate in their
           activities" [syn: clannishly, cliquishly]