Shoo-in (4)
I believe the answer is:
lock
'shoo-in' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lock that I've seen before include "Bolt" , "section of canal" , "temporary container for lighter, say" , "Tress of hair" , "Mechanism to keep closed, door say" .)