Mineral not totally crystal clear (4)
I believe the answer is:
talc
'mineral' is the definition.
(I know that talc is a type of mineral)
'not totally crystal clear' is the wordplay.
'not totally' says the answer is hidden in the clue (only some letters are used).
'TALC' is hidden in the letters of 'crystal clear'.
(Other definitions for talc that I've seen before include "Cosmetic powder" , "Powdered substance" , "Baby powder" , "Perfumed drying powder" , "Toilet powder - mica" .)