Nitrous oxide (8,3)
I believe the answer is:
laughing gas
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">nitrous oxide</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)