Substance in cereals (4)
I believe the answer is:
bran
'substance in cereals' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for bran that I've seen before include "Husks of corn" , "Milled grain husks" , "Separated grain husks" , "Dietary fibre source" , "Health food" .)