Powder used in experimental chemistry (4)
I believe the answer is:
talc
'powder' is the definition.
(talcing is a kind of powdering)
'in experimental chemistry' is the wordplay.
'in' indicates the answer is hidden within the clue.
'TALC' is hidden within 'experimental chemistry'.
'used' acts as a link.
(Other definitions for talc that I've seen before include "Soft mineral; powder" , "In brief, popular body powder" , "White powder for the bathroom" , "Scented powder used after bathing" , "It is used to absorb moisture" .)