Number three at sea (5)
I believe the answer is:
ether
'number' is the definition.
(anaesthetic, something which numbs)
'three at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'three' with letters rearranged gives 'ETHER'.
(Other definitions for ether that I've seen before include "It could knock you out" , "Anaesthetic liquid; clear sky" , "Once widely-used anaesthetic" , "Rare atmosphere" , "Formerly an anaesthetic" .)