Material behind original cooked English breakfast (6)
I believe the answer is:
cereal
'breakfast' is the definition.
(I've seen this before)
'material behind original cooked english' is the wordplay.
'material' becomes 'real' (associated in meaning).
'behind' is a charade indicator (letters next to each other).
'original' suggests taking the first letters.
The initial letters of 'cooked english' is 'ce'.
'real' put after 'ce' is 'CEREAL'.
(Other definitions for cereal that I've seen before include "Grain used for food" , "Grass cultivated for food" , "Eg, wheat, rye, corn" , "Grass producing an edible grain" , "Breakfast food made from grain" .)