Case of champagne — genuine breakfast option (6)
I believe the answer is:
cereal
'breakfast option' is the definition.
(I've seen this in another clue)
'case of champagne genuine' is the wordplay.
'case of' means to remove the middle letters (like emptying a case).
'genuine' becomes 'real' (associated in meaning).
'champagne' with its centre removed is 'ce'.
'ce'+'real'='CEREAL'
(Other definitions for cereal that I've seen before include "Common part of breakfast" , "11 food, maybe" , "Food prepared from grain" , "Maize, rye or the like" , "Relating to grain" .)