The Collaborative International Dictionary of English v.0.48:

Clash \Clash\, v. i. [imp. & p. p. Clashed; p. pr. & vb. n.
   Clashing.] [Of imitative origin; cf. G. klatschen, Prov. G.
   kleschen, D. kletsen, Dan. klaske, E. clack.]
   1. To make a noise by striking against something; to dash
      noisily together.
      [1913 Webster]

   2. To meet in opposition; to act in a contrary direction; to
      come onto collision; to interfere.
      [1913 Webster]

            However some of his interests might clash with those
            of the chief adjacent colony.         --Palfrey.
      [1913 Webster]

WordNet (r) 3.0 (2006):

clashing
    adj 1: sharply and harshly discordant; "clashing interests of
           loggers and conservationists"; "clashing colors"

Moby Thesaurus II by Grady Ward, 1.0:

129 Moby Thesaurus words for "clashing":
   acrid, adversary, adversative, adverse, adversive, ajar, alien,
   antagonistic, anti, antipathetic, antithetic, antonymous,
   at cross-purposes, at loggerheads, at odds, at variance, at war,
   balancing, belligerent, bitter, breakaway, caustic, colliding,
   compensating, competitive, con, conflicting, confronting, confused,
   contradicting, contradictory, contradistinct, contrapositive,
   contrarious, contrary, contrasted, converse, counter,
   counteractant, counteracting, counteractive, counterbalancing,
   counterpoised, countervailing, counterworking, cranky, cross,
   crotchety, dead against, despiteful, differing, disaccordant,
   disagreeable, disagreeing, discordant, discrepant, disharmonious,
   disproportionate, dissentient, dissident, dissonant, divergent,
   enemy, eyeball to eyeball, fractious, full of hate, grating, harsh,
   hateful, hostile, immiscible, inaccordant, incompatible,
   incongruous, inconsistent, inharmonious, inimical, inverse,
   jangling, jangly, jarring, jostling, malevolent, malicious,
   malignant, negative, nonconformist, noncooperative, obstinate,
   obverse, off-color, off-tone, opponent, opposed, opposing,
   opposite, oppositional, oppositive, oppugnant, out of accord,
   out of whack, overthwart, perverse, quarrelsome, rancorous,
   reactionary, recalcitrant, refractory, renitent, repugnant,
   resistant, reverse, revolutionary, rival, set against, sore,
   spiteful, squared off, uncongenial, uncooperative, unfavorable,
   unfriendly, unharmonious, unpropitious, variant, venomous,
   virulent, vitriolic, warring