A number - three, perhaps (5)
I believe the answer is:
ether
'a number' is the definition.
(anaesthetic, something which numbs)
'three perhaps' is the wordplay.
'perhaps' indicates an anagram.
'three' anagrammed gives 'ETHER'.
(Other definitions for ether that I've seen before include "Anaesthetic is, strangely, there" , "Anaesthetic; clear sky (poetic)" , "number" , "Old anaesthetic -- three (anag)" , "Highly flammable liquid once used as an anaesthetic" .)