Day divider (4)
I believe the answer is:
noon
'day divider' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for noon that I've seen before include "Twelve o'clock in the day" , "Twelve during daylight" , "Twelve o'clock, midday" , "12 pm" , "The middle of the day" .)