Fasten hair (4)
I believe the answer is:
lock
This clue is a double definition.
'fasten' is the first definition.
(I know that lock is a type of fasten)
'hair' is the second definition.
(I know that lock is a type of hair)
(Other definitions for lock that I've seen before include "Tress of hair" , "Key in this" , "Rugby player" , "Security device - tuft of hair" , "Strand(s) of hair" .)