See back covered in adhesive plaster (6)
I believe the answer is:
gypsum
'plaster' is the definition.
(mineral used to make plaster)
'see back covered in adhesive' is the wordplay.
'see' becomes 'spy' (spy can mean to see or observe).
'back' is a reversal indicator.
'covered in' means one lot of letters goes inside another.
'adhesive' becomes 'gum' (gum is a kind of adhesive).
'spy' written backwards gives 'yps'.
'yps' placed within 'gum' is 'GYPSUM'.
(Other definitions for gypsum that I've seen before include "White mineral in cement and plaster" , "Ingredient of fertilizer" , "Plaster of Paris is made from this chalk-like mineral" , "that provides material for casts" , "Chalky mineral from which plaster of Paris is made" .)