Dental assistant tried noxious compound (7,5)
I believe the answer is:
nitrous oxide
'dental assistant' is the definition.
(sedative used in dentistry)
'tried noxious compound' is the wordplay.
'compound' is an anagram indicator.
'tried'+'noxious'='triednoxious'
'triednoxious' with letters rearranged gives 'NITROUS OXIDE'.
(Other definitions for nitrous oxide that I've seen before include "that will give you a laugh" , "a soporific" , "Laughing gas" , "that should make you laugh!" , "Sweet-smelling gas, N2O" .)