Number three fouled (5)
I believe the answer is:
ether
'number' is the definition.
(anaesthetic, something which numbs)
'three fouled' is the wordplay.
'fouled' is an anagram indicator.
'three' with letters rearranged gives 'ETHER'.
(Other definitions for ether that I've seen before include "It could knock you out" , "Clear upper air" , "Colourless, highly flammable liquid" , "Once widely-used anaesthetic" , "Former anaesthetic" .)