Upper atmosphere (5)
I believe the answer is:
ether
'upper atmosphere' 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 "Colourless volatile highly inflammable liquid" , "Old anaesthetic -- three (anag)" , "Highly flammable liquid once used as an anaesthetic" , "Light liquid used as anaesthetic" , "Solvent; clear sky (poet.)" .)