Anaesthetic used in gene therapy (5)
I believe the answer is:
ether
'anaesthetic' is the definition.
(ether is a kind of anaesthetic)
'in gene therapy' is the wordplay.
'in' indicates a hidden word.
'ETHER' is hidden within 'gene therapy'.
'used' acts as a link.
(Other definitions for ether that I've seen before include "Old anaesthetic -- three (anag)" , "Colourless liquid used as a solvent" , "Colourless, highly flammable liquid" , "Colourless volatile highly inflammable liquid" , "Volatile solvent" .)