Mineral stored in metal container (4)
I believe the answer is:
talc
'mineral' is the definition.
(I know that talc is a type of mineral)
'stored in metal container' is the wordplay.
'stored in' says the answer is hidden in the clue.
'TALC' can be found hidden inside 'metal container'.
(Other definitions for talc that I've seen before include "Soft white powder" , "Fine powder" , "Softest mineral on Mohs hardness scale" , "Toilet powder - mica" , "Softest substance in Mohs scale of hardness" .)