Anaesthetic extract of sweet herbs (5)
I believe the answer is:
ether
'anaesthetic' is the definition.
(ether is a kind of anaesthetic)
'extract of sweet herbs' is the wordplay.
'extract of' indicates the answer is hidden within the clue (I've seen this in other clues).
'ETHER' is hidden within 'sweet herbs'.
(Other definitions for ether that I've seen before include "Some were put out by this" , "Formerly an anaesthetic" , "Colourless, highly flammable liquid" , "Rare atmosphere" , "Gas - air - space" .)