The Collaborative International Dictionary of English v.0.48:

Clench \Clench\, n. & v. t.
   See Clinch.
   [1913 Webster]

WordNet (r) 3.0 (2006):

clench
    n 1: a small slip noose made with seizing [syn: clinch,
         clench]
    2: the act of grasping; "he released his clasp on my arm"; "he
       has a strong grip for an old man"; "she kept a firm hold on
       the railing" [syn: clasp, clench, clutch, clutches,
       grasp, grip, hold]
    v 1: hold in a tight grasp; "clench a steering wheel" [syn:
         clench, clinch]
    2: squeeze together tightly; "clench one's jaw"

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "clench":
   abduct, adhere to, bear hug, bite, carry off, clamp, clasp,
   cleave to, clinch, cling, clinging, clip, clutch, death grip,
   embrace, firm hold, foothold, footing, freeze to, grapple, grasp,
   grip, gripe, hang on, hang on to, hold, hold fast, hold on,
   hold on to, hold tight, hug, iron grip, keep hold of, kidnap,
   never let go, nip, purchase, seizure, shanghai, stick to, tenure,
   throttle, tight grip, toehold