Article used in anaesthesia (3)
I believe the answer is:
the
'article' is the definition.
(the definite article in English)
'in anaesthesia' is the wordplay.
'in' indicates the answer is hidden within the clue.
'THE' is hidden in the letters of 'anaesthesia'.
'used' acts as a link.
(Other definitions for the that I've seen before include "French drink" , "See 10 Across" , "Born - - - - - - - - (carol)" , "and 24 Down: Similar things" , "Over - - moon" .)