Inorganic substance (7)
I believe the answer is:
mineral
'inorganic substance' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mineral that I've seen before include "Substance that is (usually) inorganic" , "Metal ore" , "Substance needed for health; ore" , "Gypsum or magnesia, say" , "Zinc or magnesium, say" .)