Poisonous vapour avengers spread (5,3)
I believe the answer is:
nerve gas
'poisonous vapour' is the definition.
(I've seen this before)
'avengers spread' is the wordplay.
'spread' indicates anagramming the letters.
'avengers' with letters rearranged gives 'NERVE GAS'.
(Other definitions for nerve gas that I've seen before include "Dangerous concoction" , "Type of weapon" , "Poison" , "Poisonous vapour" , "Chemical warfare agent" .)