It's initially explosive (3)
I believe the answer is:
tnt
'explosive' is the definition.
(I know that TNT is a type of explosive compound)
'it's initially' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'initially' becomes 'nt' (I am not sure about this - if you are sure you should believe this answer much more).
't'+'nt'='TNT'
Can you help me to learn more?
(Other definitions for tnt that I've seen before include "Explosive compound" , "Very powerful explosive, compressed" , "Type of high explosive" , "Explosive stuff" , "Initially, powerful explosive" .)