The term 'certain fastener' can refer to a variety of items, including a CLASP (5 letters), a SCREW (5 letters), a u-bolt (5 letters), and a safety pin (9 letters). Clasps are two pieces of metal that are joined together by a hinge, and are often used to secure jewelry or clothing. Screws are cylindrical pieces of metal with ridges on the outside, and are used to join two objects together. U-bolts are U-shaped pieces of metal with threads on the outside, and are used to secure pipes or other objects. Safety pins are small pins with a spring-loaded closure, and are used to secure fabric or clothing.