Witch wearing make up perhaps important element of diet (8)
I believe the answer is:
roughage
'important element of diet' is the definition.
(roughage means food containing fibre)
'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' going within 'rouge' is 'ROUGHAGE'.
'perhaps' acts as a link.
(Other definitions for roughage that I've seen before include "Bulk" , "something that can make your passing more bearable" , "Vegetable fibre" , "Dietary fibre" , "Healthy fare" .)