Porridge grain (4)
'porridge grain' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for oat that I've seen before include "Cereal grass" , "Cereal plant" , "Food grain" , "Cereal (singular)" , "Basis of porridge" .)
(Other definitions for oats that I've seen before include "Porridge mix" , "Basis of porridge" , "Grains or porridge" , "Sow wild ones in disreputable youth" , "Cereal plants" .)