Part of a lock (4)
I believe the answer is:
bolt
'part of a lock' is the definition.
('bolt' is part of 'lock')
This is all the clue.
(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" .)