Powder hidden by the ancient alchemists (4)
I believe the answer is:
talc
'powder' is the definition.
(I know this)
'hidden by the ancient alchemists' is the wordplay.
'hidden' indicates a hidden word.
'TALC' is hidden within 'by ancient alchemists'.
(Other definitions for talc that I've seen before include "(Mineral providing) powder" , "Softest mineral on Mohs hardness scale" , "Softest substance in Mohs scale of hardness" , "Powdered substance" , "Bathroom powder (abbr)" .)