Anaesthetic is article in emergency room (5)
I believe the answer is:
ether
'anaesthetic' is the definition.
(ether is a kind of anaesthetic)
'article in emergency room' is the wordplay.
'article' becomes 'the' (the definite article in English).
'in' indicates putting letters inside.
'emergency room' becomes 'ER' (abbreviation popularized by US TV series).
'the' placed within 'er' is 'ETHER'.
'is' is the link.
(Other definitions for ether that I've seen before include "number" , "Former anaesthetic" , "It could knock you out" , "There goes the anaesthetic" , "Light liquid used as anaesthetic" .)