Nitrous oxide (8,3)
I believe the answer is:
laughing gas
'nitrous oxide' is the definition.
(synonyms)
This is all the clue.
(Other definitions for laughing gas that I've seen before include "Mild anaesthetic used in dentistry and surgery" , "it'll knock you out" , "Name for nitrous oxide" , "Early anaesthetic; nitrous oxide" , "Old number" .)