The Collaborative International Dictionary of English v.0.48:

Clan \Clan\ (kl[a^]n), n. [Gael. clann offspring, descendants;
   akin to Ir. clann, cland, offspring, tribe, family; perh.
   from L. plania scion, slip, cutting. Cf. Plant, n.]
   1. A tribe or collection of families, united under a
      chieftain, regarded as having the same common ancestor,
      and bearing the same surname; as, the clan of Macdonald.
      "I have marshaled my clan." --Campbell.
      [1913 Webster]

   2. A clique; a sect, society, or body of persons; esp., a
      body of persons united by some common interest or pursuit;
      -- sometimes used contemptuously.
      [1913 Webster]

            Partidge and the rest of his clan may hoot me.
                                                  --Smolett.
      [1913 Webster]

            The whole clan of the enlightened among us. --Burke.
      [1913 Webster]

WordNet (r) 3.0 (2006):

clan
    n 1: group of people related by blood or marriage [syn: kin,
         kin group, kinship group, kindred, clan, tribe]

Moby Thesaurus II by Grady Ward, 1.0:

141 Moby Thesaurus words for "clan":
   animal kingdom, ashram, band, blood, body, bracket, branch, brand,
   breed, brood, brotherhood, bunch, cabal, cadre, camarilla, camp,
   cast, caste, category, cell, character, charmed circle, circle,
   class, clique, closed circle, colony, color, commonwealth, commune,
   community, coterie, crew, crowd, deme, denomination, description,
   designation, division, dynasty, economic class, elite, elite group,
   endogamous group, estate, extended family, faction, family,
   feather, fellowship, folk, form, fraternity, gang, genre, gens,
   genus, grade, grain, group, grouping, head, heading, house, ilk,
   ingroup, inner circle, junta, junto, kidney, kin, kind, kindred,
   kinship group, label, level, line, lineage, lot, make, manner,
   mark, matriclan, mob, moiety, mold, nation, nature, nuclear family,
   number, order, outfit, party, patriclan, people, persuasion,
   phratria, phratry, phyle, phylum, pigeonhole, plant kingdom,
   position, predicament, race, rank, rating, ring, rubric, section,
   sept, set, settlement, shape, sly, social class, society, sort,
   species, stamp, station, status, stem, stirps, stock, strain,
   stratum, stripe, style, subcaste, subdivision, subgroup, suborder,
   the like of, the likes of, title, totem, tribe, type, variety,
   we-group