Day and night (5,3,5)
I believe the answer is:
round the clock
'day and night' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for round the clock that I've seen before include "unbroken shifts?" , "At all times of day or night" , "As hands work" , "24 hours without a break" , "What some dart players play" .)