Anaesthetic used in some therapy (5)
I believe the answer is:
ether
'anaesthetic' is the definition.
(ether is a kind of anaesthetic)
'in some therapy' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ETHER' is hidden within 'some therapy'.
'used' is the link.
(Other definitions for ether that I've seen before include "Gas - air - space" , "Volatile solvent" , "Colourless, highly flammable liquid" , "Volatile organic liquid once used as a general anaesthetic" , "Highly flammable liquid once used as an anaesthetic" .)