Breakfast's beginning with terribly early cereal (6)
I believe the answer is:
barley
'cereal' is the definition.
(I know that barley is a type of cereal)
'breakfast's beginning with terribly early' is the wordplay.
'beginning' indicates taking the first letters.
'with' is a charade indicator (letters next to each other).
'terribly' is an anagram indicator.
The initial letter of 'breakfasts' is 'b'.
'early' with letters rearranged gives 'arley'.
'b'+'arley'='BARLEY'
(Other definitions for barley that I've seen before include "Cereal used in beer" , "Cereal crop" , "Cereal used in brewing" , "Grain used in brewing" , "Type of cereal" .)