Anaesthetic; air (5)
I believe the answer is:
ether
I believe this clue is a double definition.
'anaesthetic' is the first definition.
(ether is a kind of anaesthetic)
'air' is the second definition.
(literary term for the air or sky)
(Other definitions for ether that I've seen before include "Imaginary interstellar medium" , "Liquid used as anaesthetic/ solvent" , "Clear upper air" , "Some were put out by this" , "Colourless volatile highly inflammable liquid" .)