Cereal bit (4)
'cereal bit' 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 plant" , "Cereal grass" , "Cereal (singular)" , "Basis of porridge" , "Food grain" .)
(Other definitions for bran that I've seen before include "Health food" , "Husks after milling" , "Refuse from grain" , "Fibre cereal" , "Coarse meal for horse food" .)