Revile harmful gas American imported (4)
I believe the answer is:
cuss
'revile' is the definition.
(cuss can mean to curse or speak badly of)
'harmful gas american imported' is the wordplay.
'harmful gas' becomes 'cs' (short for 2-chlorobenzalmalononitrile. I am not sure about the 'harmful' bit.).
'american' becomes 'us' (abbreviation for United States).
'imported' means one lot of letters goes inside another.
'cs' enclosing 'us' is 'CUSS'.
Can you help me to learn more?
(Other definitions for cuss that I've seen before include "Swear (slang)" , "Stubborn person" , "Oath; (annoying sort of) person" , "Use bad language (informal)" , "Blaspheme" .)