Cosmos; an anaesthetic (5)
I believe the answer is:
ether
I believe this is a double definition.
'cosmos' is the first definition.
(I've seen this in another clue)
'an anaesthetic' is the second definition.
(ether is a kind of anaesthetic)
(Other definitions for ether that I've seen before include "An early anaesthetic" , "Rare atmosphere" , "Type of chemical" , "Old anaesthetic -- three (anag)" , "Anaesthetic is, strangely, there" .)