Shot in back procuring a toxic gas (7)
I believe the answer is:
ammonia
'toxic gas' is the definition.
(I know that ammonia is toxic)
'shot in back procuring a' is the wordplay.
'shot' becomes 'ammo' (ammunition).
'back' says the letters should be written backwards.
'procuring' means one lot of letters go next to another.
'in' back-to-front is 'ni'.
'ammo'+'ni'+'a'='AMMONIA'
(Other definitions for ammonia that I've seen before include "Pungent cleaning fluid" , "Colourless gas with strong smell" , "Colourless pungent gas" , "Pungent gas, NH 3" , "Powerful cleaner" .)