Former anesthetic (5)
I believe the answer is:
ether
'former anesthetic' 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 "Volatile solvent" , "Volatile organic liquid once used as a general anaesthetic" , "An early anaesthetic" , "Anaesthetic; clear sky (poetic)" , "Former anaesthetic" .)