Inert compound becoming explosive (5)
I believe the answer is:
nitre
'explosive' is the definition.
(I've seen this before)
'inert compound' is the wordplay.
'compound' is an anagram indicator.
'inert' anagrammed gives 'NITRE'.
'becoming' is the link.
(Other definitions for nitre that I've seen before include "what goes into bangers" , "fertiliser" , "Inert kind of saltpetre" , "found in a slow match?" , "explosive producer?" .)