Anesthetic of old (5)
I believe the answer is:
ether
'anesthetic of old' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ether that I've seen before include "Chemical solvent" , "Colourless, highly flammable liquid" , "Colourless liquid used as a solvent" , "It could knock you out" , "Colourless volatile highly inflammable liquid" .)