Saltpetre (5)
I believe the answer is:
nitre
'saltpetre' is the definition.
(I know that saltpetre can be written as nitre)
This is the entire clue.
(Other definitions for nitre that I've seen before include "constituent of gunpowder" , "Also known as salt-petre" , "explosive producer?" , "Inert kind of saltpetre" , "Chemical" .)