Anaesthetic that’s used there (5)
I believe the answer is:
ether
'anaesthetic' is the definition.
(ether is a kind of anaesthetic)
'used there' is the wordplay.
'used' is an anagram indicator.
'there' with letters rearranged gives 'ETHER'.
'that's' acts as a link.
(Other definitions for ether that I've seen before include "Former anaesthetic" , "What's clear" , "It could knock you out" , "The gas is there, oddly" , "Anaesthetic is, strangely, there" .)