It's initially explosive (3)
I believe the answer is:
tnt
'explosive' is the definition.
(TNT is an example)
'it's initially' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'initially' becomes 'nt' (I can't justify this - if you can 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" .)