Day's opposite (5)
I believe the answer is:
night
'day's opposite' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for night that I've seen before include "Sunset to sunrise" , "What proverbially follows day" , "Day's partner" , "That's when it's darkest" , "follow-up to 16 [TWILIGHT]" .)