Ingredient in plaster put over spot in face (6)
I believe the answer is:
gypsum
'ingredient in plaster' is the definition.
(plaster of Paris contains gypsum)
'over spot in face' is the wordplay.
'over' shows that the letters should be reversed in order.
'spot' becomes 'spy' (synonyms).
'in' indicates putting letters inside.
'face' becomes 'mug' (informal term for face).
'spy' put inside 'mug' is 'muspyg'.
'muspyg' written backwards gives 'GYPSUM'.
'put' acts as a link.
(Other definitions for gypsum that I've seen before include "Soft white or grey mineral" , "Plaster of Paris is made from this chalk-like mineral" , "Base of plaster of Paris" , "Alabaster" , "Ingredient of plaster of Paris" .)