Breakfast foods (7)
I believe the answer is:
'breakfast foods' is the definition.
(cereal is a kind of breakfast food)
This is all the clue.
(Other definitions for cereals that I've seen before include "Foods such as rye, wheat, etc" , "Stubbornly unmoving" , "Breakfast foods" , "Starchy grains used as food" , "Grain foods" .)