Old anaesthetic (5)
I believe the answer is:
ether
'old anaesthetic' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ether that I've seen before include "Anaesthetic liquid; clear sky" , "Colourless liquid used as a solvent" , "I'm volatile" , "Gas - air - space" , "Solvent; clear sky (poet.)" .)