Inert form of explosive ingredient (5)
I believe the answer is:
nitre
'explosive ingredient' is the definition.
(I've seen this before)
'inert form' is the wordplay.
'form' indicates an anagram (another form of the letters).
'inert' with letters rearranged gives 'NITRE'.
'of' acts as a link.
(Other definitions for nitre that I've seen before include "Chemical" , "explosive component" , "fertiliser" , "explosive ingredient" , "found in a slow match?" .)