Protest carbon monoxide from motorway on grassland (8)
I believe the answer is:
complain
'protest' is the definition.
(I know that protest is a more specific form of the action complain)
'carbon monoxide from motorway on grassland' is the wordplay.
'carbon monoxide' becomes 'CO' (chemical formula for carbon monoxide).
'from' is a charade indicator (letters next to each other).
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'on' says to put letters next to each other.
'grassland' becomes 'plain' (I have seen 'Open grassland' mean 'plain' so perhaps 'grassland' could also mean 'plain').
'co'+'m'+'plain'='COMPLAIN'
(Other definitions for complain that I've seen before include "Express a sense of injury or say you're ill" , "Beef (or grouse?)" , "Express discontent" , "Find fault" , "Remonstrate" .)