Cereal fibre (4)
I believe the answer is:
bran
'cereal fibre' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for bran that I've seen before include "Health food" , "Husks of grain sifted from the flour" , "Refuse from grain" , "Pieces of grain husks" , "Husks of cereal seeds sifted from flour" .)