Sample of crystal clear powder (4)
I believe the answer is:
talc
'powder' is the definition.
(talcing is a kind of powdering)
'sample of crystal clear' is the wordplay.
'sample of' says the answer is hidden in the clue (I've seen this in other clues).
'TALC' can be found hidden inside 'crystal clear'.
(Other definitions for talc that I've seen before include "soother" , "Softest substance in Mohs scale of hardness" , "Fine powder" , "Powdered substance" , "Cosmetic powder" .)