Onetime anesthetic (5)
I believe the answer is:
ether
'onetime anesthetic' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ether that I've seen before include "Gas - air - space" , "I'm volatile" , "Formerly an anaesthetic" , "Colourless volatile highly inflammable liquid" , "Type of chemical" .)