Early number? (5)
I believe the answer is:
ether
'early number?' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ether that I've seen before include "Old anaesthetic -- three (anag)" , "There goes the anaesthetic" , "It could knock you out" , "The gas is there, oddly" , "Formerly an anaesthetic" .)