Compound used in gene therapy (5)
I believe the answer is:
ether
'compound' is the definition.
(chemical compound)
'in gene therapy' is the wordplay.
'in' indicates a hidden word.
'ETHER' is hidden in the letters of 'gene therapy'.
'used' acts as a link.
(Other definitions for ether that I've seen before include "The gas is there, oddly" , "Highly flammable liquid once used as an anaesthetic" , "Imaginary interstellar medium" , "Anaesthetic is, strangely, there" , "It could knock you out" .)