Number three sent off (5)
I believe the answer is:
ether
'number' is the definition.
(anaesthetic, something which numbs)
'three sent off' is the wordplay.
'sent off' is an anagram indicator.
'three' with letters rearranged gives 'ETHER'.
(Other definitions for ether that I've seen before include "Clear upper air" , "Former anaesthetic" , "Volatile organic liquid once used as a general anaesthetic" , "There goes the anaesthetic" , "Some were put out by this" .)