Vital chemical — powder hidden in that (4)
I believe the answer is:
talc
'in that' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'vital chemical powder hidden' is the wordplay.
'hidden' indicates the answer is hidden within the clue.
'TALC' is hidden within 'vital chemical powder'.
(Other definitions for talc that I've seen before include "Bathroom powder (abbr)" , "Softest mineral on Mohs hardness scale" , "In brief, popular body powder" , "Bathroom dusting powder" , "I may dust" .)