When you unclasp something, you open a fastening or loosen a grip. You should unclasp your gold locket and leave it someplace safe before you go surfing.
Undo a clasp, a fastener or small lock, and you unclasp it. So you might unclasp your seatbelt once the plane has landed, or unclasp your bracelet so that your best friend can try it on. You can also undo another kind of clasp — a hug, hold, or embrace: "I missed her so much that I didn't ever want to unclasp my fingers from hers once she came home."