Extremely coarse, genuine breakfast food? (6)
I believe the answer is:
cereal
'breakfast food?' is the definition.
(cereal is a kind of breakfast food)
'extremely coarse genuine' is the wordplay.
'extremely' means to remove the middle letters.
'genuine' becomes 'real' ('real' is associated in meaning with 'genuine').
'coarse' with its centre removed is 'ce'.
'ce'+'real'='CEREAL'
(Other definitions for cereal that I've seen before include "Eg, 5 Ac" , "Breakfast dish" , "Rye, for example" , "Grain used for food" , "Eg, wheat, rice" .)