Extremely coarse, proper grass (6)
I believe the answer is:
cereal
'grass' is the definition.
(thesaurus)
'extremely coarse proper' is the wordplay.
'extremely' says to hollow out the word (remove centre letters).
'proper' becomes 'real' ('real' can be similar in meaning to 'proper').
'coarse' with its centre taken out is 'ce'.
'ce'+'real'='CEREAL'
(Other definitions for cereal that I've seen before include "Common part of breakfast" , "Grass cultivated for food" , "Grass producing an edible grain" , "Rye, for example" , "Grain crop" .)