Head lock (5)
I believe the answer is:
tress
'head lock' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tress that I've seen before include "Long piece of hair" , "Rests (anag.)" , "Braid of hair" , "Long lock or plait of hair" , "Obsolete support" .)