Secure 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)
'hair' is the second definition.
(lock is a kind of hair)
(Other definitions for lock that I've seen before include "Hold < wisp of wool" , "Bolt" , "part of 10 [CANAL]" , "Become immoveable" , "Security or navigational device" .)