To prise or force open (a person's tightly closed hand, a grip, etc.); to cause to open; = "unclench".

To relax or open (one's tightly closed hand, fist, etc.); = "unclench".

And reflexive. Of a tightly closed hand, fist, etc.: to relax; to open; = "unclench".

To loosen (a nail, especially one used to attach a horseshoe) by prising up the point, when this has been bent over or flattened to secure it. Formerly also: †to remove the nails from (a horseshoe) (obsolete).