"High" time of day (4)
I believe the answer is:
noon
'high time of day' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for noon that I've seen before include "Twelve o'clock, midday" , "A moment of time" , "when only mad dogs go out?" , "High time of day?" , "Twelve o'clock in the day" .)