Old number three fouled (5)
I believe the answer is:
ether
'old number' is the definition.
(chemical historically used to numb patients)
'three fouled' is the wordplay.
'fouled' indicates an anagram.
'three' with letters rearranged gives 'ETHER'.
(Other definitions for ether that I've seen before include "Once widely-used anaesthetic" , "Medium supposedly" , "Anaesthetic liquid; clear sky" , "There goes the anaesthetic" , "The gas is there, oddly" .)