Old anaesthetic used in some therapy (5)
I believe the answer is:
ether
'old anaesthetic' is the definition.
(I've seen this before)
'in some therapy' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ETHER' is hidden in the letters of 'some therapy'.
'used' acts as a link.
(Other definitions for ether that I've seen before include "Some were put out by this" , "Clear upper air" , "Medium supposedly" , "Once widely-used anaesthetic" , "Colourless liquid used as a solvent" .)