Fiber source (4)
I believe the answer is:
bran
'fiber source' 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 "Coarse meal for horses" , "Dietary fibre source" , "Pieces of grain husks" , "Element in high fibre diet" , "Horse food stored in barn" .)