Fastener for hair (4)
I believe the answer is:
lock
This clue is a double definition.
'fastener' is the first definition.
(I know that lock is a type of fastener)
'for hair' is the second definition.
(a lock of hair)
(Other definitions for lock that I've seen before include "element in pack" , "part of 10 [CANAL]" , "Tress of hair" , "Mechanism to keep closed, door say" , "tuft of wool" .)