Gas in cannon exploded backwards (5)
I believe the answer is:
xenon
'gas' is the definition.
(thesaurus)
'in cannon exploded backwards' is the wordplay.
'in' says the answer is hidden in the clue.
'backwards' is a reversal indicator.
'nonex' can be found hidden inside 'cannon exploded'.
'nonex' in reverse letter order is 'XENON'.
(Other definitions for xenon that I've seen before include "Inert gas used in radio valves" , "member of nobility?" , "Chemical element" , "Inert gas, Xe" , "Noble element" .)