Clear-sky color (4)
I believe the answer is:
blue
'clear-sky color' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for blue that I've seen before include "down in the dumps" , "Sad; university sportsman" , "Miserable - cerulean" , "ball at snooker" , "This could be royal or navy" .)