Powder used by first alchemists (4)
I believe the answer is:
talc
'powder' is the definition.
(I know that talc is a type of powder)
'used by first alchemists' is the wordplay.
'used by' indicates a hidden word.
'TALC' can be found hidden inside 'first alchemists'.
(Other definitions for talc that I've seen before include "Drier" , "White powder for the bathroom" , "Soft stuff" , "Scented powder used after bathing" , "Cosmetic powder" .)