Mineral dial surrounding watch, reflective (6)
I believe the answer is:
gypsum
'mineral' is the definition.
(I know that gypsum is a type of mineral)
'dial surrounding watch reflective' is the wordplay.
'dial' becomes 'mug' (both can informally mean someone's face).
'surrounding' indicates putting letters inside.
'watch' becomes 'spy' (I've seen this in another clue).
'reflective' says the letters should be written backwards.
'mug' placed around 'spy' is 'muspyg'.
'muspyg' back-to-front is 'GYPSUM'.
(Other definitions for gypsum that I've seen before include "that provides material for casts" , "White mineral in cement and plaster" , "Mineral used in making plaster of Paris" , "Ingredient of plaster of Paris" , "Mineral giving plaster of Paris" .)