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"