Clock or watch (5)
I believe the answer is:
timer
I believe this clue is a double definition.
'clock' is the first definition.
(something which times)
'watch' is the second definition.
(I've seen this before)
(Other definitions for timer that I've seen before include "Clocklike actuator" , "Trigger mechanism" , "Clock or watch" , "Maybe watch" , "hourglass" .)