Strand of hair (4)
I believe the answer is:
lock
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">strand of hair</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for lock that I've seen before include "element in pack" , "Key in this" , "Secure a piece of hair" , "tuft of wool" , "Canal raiser; secure" .)