The Collaborative International Dictionary of English v.0.48:

Unclench \Un*clench"\, v. t.
   Same as Unclinch.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Unclinch \Un*clinch"\, v. t. [1st pref. un- + clinch.]
   To cause to be no longer clinched; to open; as, to unclinch
   the fist. [Written also unclench.]
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "unclench":
   bare, clear, deobstruct, disclose, expose, free, manifest, patefy,
   reveal, unblock, uncase, unclog, unclutch, uncork, uncover,
   uncurtain, undo, undrape, unfold, unfoul, unlatch, unlock, unplug,
   unroll, unseal, unsheathe, unshut, unstop, unveil, unwrap