Oscar sitting down to eat cereal (7)
I believe the answer is:
oatmeal
'cereal' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for oatmeal that I've seen before include "Ground grain; colour" , "Ground cereal" , "Basis of porridge" , "ground food!" , "Greyish-fawn colour flecked with brown" .)