Secure a bit of the hair (4)
I believe the answer is:
lock
This might be a double definition.
'secure a bit' is the first definition.
'lock' can be an answer for 'secure' (locking is a kind of securing). I'm unsure of the 'a bit' bit.
'the hair' is the second definition.
(a lock of hair)
'of' acts as a link.
Can you help me to learn more?
(Other definitions for lock that I've seen before include "Clasp" , "Get stuck in" , "Hold < wisp of wool" , "section of canal" , "temporary container for lighter, say" .)