Pieces of grain husk (4)
I believe the answer is:
bran
'pieces of grain husk' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for bran that I've seen before include "Husks valued as fibre" , "Horse-food in barn" , "Refuse providing fibre" , "Coarse meal for horse food" , "Type of edible fibre" .)