One demonstrated a crystalline compound (6)
I believe the answer is:
isatin
'a crystalline compound' is the definition.
'one demonstrated' is the wordplay.
'one' becomes 'i' (roman numeral).
'demonstrated' becomes 'satin' (as in held a sit-in, a kind of demonstration).
'i'+'satin'='ISATIN'
(Other definitions for isatin that I've seen before include "chemical derived from indigo" , "Dye ingredient" , "compound" .)