Former anaesthetic (5)
I believe the answer is:
ether
'former anaesthetic' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ether that I've seen before include "Type of chemical" , "Clear upper air" , "Liquid used as anaesthetic/ solvent" , "Highly flammable liquid once used as an anaesthetic" , "What's clear" .)