The Collaborative International Dictionary of English v.0.48:

Clinch \Clinch\ (kl[i^]nch; 224), v. t. [imp. & p. p.
   Clinched; p. pr. & vb. n. Clinching.] [OE. clenchen,
   prop. causative of clink to cause to clink, to strike; cf. D.
   klinken to tinkle, rivet. See Clink.]
   1. To hold firmly; to hold fast by grasping or embracing
      tightly. "Clinch the pointed spear." --Dryden.
      [1913 Webster]

   2. To set closely together; to close tightly; as, to clinch
      the teeth or the first. --Swift.
      [1913 Webster]

   3. To bend or turn over the point of (something that has been
      driven through an object), so that it will hold fast; as,
      to clinch a nail.
      [1913 Webster]

   4. To make conclusive; to confirm; to establish; as, to
      clinch an argument. --South.
      [1913 Webster]

WordNet (r) 3.0 (2006):

clinched
    adj 1: closed or squeezed together tightly; "a clenched fist";
           "his clenched (or clinched) teeth" [syn: clenched,
           clinched]