Nothing at breakfast, perhaps, but cereal (7)
I believe the answer is:
oatmeal
'cereal' is the definition.
(thesaurus)
'nothing at breakfast perhaps' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'breakfast perhaps' becomes 'meal' (breakfast is a kind of meal).
'o'+'at'+'meal'='OATMEAL'
'but' acts as a link.
(Other definitions for oatmeal that I've seen before include "Basis of porridge" , "Ground cereal" , "The stuff porridge is made from" , "Pale variegated brown colour" , "The material for porridge" .)