Porridge or nothing at breakfast? (7)
I believe the answer is:
oatmeal
'porridge' is the definition.
('oatmeal' can be a synonym of 'porridge')
'nothing at breakfast?' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'breakfast?' becomes 'meal' (breakfast is a kind of meal).
'o'+'at'+'meal'='OATMEAL'
'or' acts as a link.
(Other definitions for oatmeal that I've seen before include "Porridge constituent" , "Porridge ingredient" , "The material for porridge" , "Greyish-fawn colour flecked with brown" , "Powdery substance eaten" .)