They're used to fasten one's hair (5)
I believe the answer is:
locks
'hair' is the definition.
(locks can mean hair)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for locks that I've seen before include "Canal features" , "Door fasteners" , "Securely fixes" , "Clusters of hair" , "Fastens - hair" .)