Part of a lock (4)
I believe the answer is:
bolt
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">part of a lock</span>' is the definition.<br/ ><span class="explanation-format__explanation">('bolt' is part of 'lock')</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for bolt that I've seen before include "Type of lightning" , "From the blue, it's a big surprise" , "Leave suddenly and in a hurry" , "Rush away - fastening bar" , "Involved in security" .)