Mineral - type I twice rejected (4)
I believe the answer is:
talc
'mineral' is the definition.
(I know that talc is a type of mineral)
'type i twice rejected' is the wordplay.
'type' becomes 'italic' (kind of lettering).
'i twice' becomes 'II'.
'rejected' is a deletion indicator.
'italic' with 'ii' taken out is 'TALC'.
(Other definitions for talc that I've seen before include "Soft stuff" , "Toilet powder - mica" , "It is used to absorb moisture" , "Bathroom powder (abbr)" , "Cosmetic powder" .)