Small hourglass (3,5)
I believe the answer is:
egg timer
'small hourglass' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for egg timer that I've seen before include "One may be set to, say, three minutes" , "Sandglass that runs for three (or four?) minutes" , "device for boiler" , "Meter gig for measuring the boiling of hen-fruit" , "Four-minute hourglass" .)