Though inert perhaps it is of growing importance (5)
I believe the answer is:
nitre
'it is of growing importance' is the definition.
The definition suggests a singular noun which matches the answer.
'though inert' is the wordplay.
'inert' anagrams to 'NITRE'.
But, I'm not sure how this is indicated.
'perhaps' acts as a link.
Can you help me to learn more?
(Other definitions for nitre that I've seen before include "Sodium nitrate" , "explosive ingredient" , "safeguard against decay" , "Inert kind of saltpetre" , "explosive producer?" .)