Clear sky, poetically (5)
I believe the answer is:
ether
'clear sky poetically' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ether that I've seen before include "Light liquid used as anaesthetic" , "Gas - air - space" , "Highly flammable liquid once used as an anaesthetic" , "Colourless volatile highly inflammable liquid" , "Formerly an anaesthetic" .)