Timer doesn’t open security device (4)
I believe the answer is:
lock
'security device' is the definition.
(I've seen this before)
'timer doesn't open' is the wordplay.
'timer' becomes 'clock'.
'doesn't open' means to remove the first letter.
'clock' with its initial letter taken away is 'LOCK'.
(Other definitions for lock that I've seen before include "Stop moving" , "detail of shock?" , "Bolt" , "Fasten securely" , "Door fastener or tuft of hair" .)