Later step developed a constituent of gunpowder (9)
I believe the answer is:
saltpetre
'a constituent of gunpowder' is the definition.
(I've seen this before)
'later step developed' is the wordplay.
'developed' is an anagram indicator.
'later'+'step'='laterstep'
'laterstep' with letters rearranged gives 'SALTPETRE'.
(Other definitions for saltpetre that I've seen before include "It's used as a fertilizer and explosive" , "Nitre" , "Gunpowder ingredient" , "Constituent of gunpowder" , "Component of gunpowder" .)