Chemical of growing importance (9)
I believe the answer is:
saltpetre
'chemical' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for saltpetre that I've seen before include "Compound" , "Potassium nitrate" , "It's used as a fertilizer and explosive" , "Component of gunpowder" , "Gunpowder ingredient" .)