Hourglass (5)
I believe the answer is:
timer
'hourglass' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for timer that I've seen before include "Eg, clock" , "Preset activator" , "Preset mechanisation to activate a device" , "Maybe watch" , "one of 14 [CUCKOO CLOCKS]" .)