Powder hidden in ornamental casket (4)
I believe the answer is:
talc
'powder' is the definition.
(talcing is a kind of powdering)
'hidden in ornamental casket' is the wordplay.
'hidden in' indicates the answer is hidden within the clue.
'TALC' is hidden in the letters of 'ornamental casket'.
(Other definitions for talc that I've seen before include "Softest substance in Mohs scale of hardness" , "It is used to absorb moisture" , "I may dust" , "After-bath powder" , "Softest mineral on Mohs hardness scale" .)