Secure bit of hair (4)
I believe the answer is:
lock
I believe this is a double definition.
'secure' is the first definition.
(locking is a kind of securing)
'bit of hair' is the second definition.
(a lock is a bunch of hair)
(Other definitions for lock that I've seen before include "tuft of wool" , "Secure - position in rugby team" , "Mechanism to keep closed, door say" , "Tress; fasten" , "Canal feature - hair piece" .)