Porridge ingredient principally offered at breakfast, say (7)
I believe the answer is:
oatmeal
'porridge ingredient' is the definition.
(I've seen this in another clue)
'principally offered at breakfast say' is the wordplay.
'principally' says to take the initial letters.
'breakfast say' becomes 'meal' (breakfast is a kind of meal).
The initial letter of 'offered' is 'o'.
'o'+'at'+'meal'='OATMEAL'
(Other definitions for oatmeal that I've seen before include "ground food!" , "Plain food" , "Greyish-fawn colour flecked with brown" , "Source of biscuits" , "Ground grain used to make porridge" .)