Fibre shown by witch wearing make-up (8)
I believe the answer is:
roughage
'fibre' is the definition.
(I've seen this before)
'witch wearing make-up' is the wordplay.
'witch' becomes 'hag' (synonyms).
'wearing' means one lot of letters goes inside another.
'make up' becomes 'rouge' (rouging is a kind of making up).
'hag' inserted within 'rouge' is 'ROUGHAGE'.
'shown by' is the link.
(Other definitions for roughage that I've seen before include "Indigestible material good for intestines" , "Healthy fare" , "Coarse food" , "something that can make your passing more bearable" , "Bulk" .)