Calm time of day (4)
I believe the answer is:
even
I believe this is a triple definition!
'calm' is the first definition.
(I've seen this in another clue)
'time' is the second definition.
(I have seen 'time of day' mean 'even' so perhaps 'time' could also mean 'even')
'of' acts as a link.
'day' is the third definition.
(I know that even is a type of day)
(Other definitions for even that I've seen before include "Level, par" , "Steady or smooth" , "Smooth, level" , "without a rise?" , "Constant" .)