The Collaborative International Dictionary of English v.0.48:
Unclutch \Un*clutch"\, v. t. [1st pref. un- + clutch.]
1. To open, as something closely shut. "Unclutch his griping
hand." --Dr. H. More.
[1913 Webster]
2. (Mech.) To disengage, as a clutch.
[1913 Webster]