Potassium nitrate (9)
I believe the answer is:
saltpetre
'potassium nitrate' is the definition.
(I know that potassium nitrate can be written as saltpetre)
This is the entire clue.
(Other definitions for saltpetre that I've seen before include "Gunpowder ingredient" , "Nitre" , "explosive component" , "Component of gunpowder" , "It's used as a fertilizer and explosive" .)