Dietary fibre (8)
I believe the answer is:
roughage
'dietary fibre' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for roughage that I've seen before include "Essential food" , "Bulk" , "Indigestible material to stimulate intenstines" , "Indigestible material good for intestines" , "Vegetable fibre" .)