Saltpetre is awfully inert (5)
I believe the answer is:
nitre
'saltpetre' is the definition.
(I know that saltpetre can be written as nitre)
'awfully inert' is the wordplay.
'awfully' indicates anagramming the letters.
'inert' is an anagram of 'NITRE'.
'is' acts as a link.
(Other definitions for nitre that I've seen before include "found in a slow match?" , "Salt" , "constituent of gunpowder" , "safeguard against decay" , "Otherwise saltpetre" .)