Insane sort of pigment (6)
I believe the answer is:
sienna
'pigment' is the definition.
(I know that sienna is a type of earth color)
'insane sort' is the wordplay.
'sort' indicates an anagram.
'insane' with letters rearranged gives 'SIENNA'.
'of' is the link.
(Other definitions for sienna that I've seen before include "Italian city and brownish pigment" , "Yellow-brown clay used in paint pigments" , "A shade" , "Brownish-yellow substance used as pigment" , "Browny-yellow pigment" .)