An explosive thing? That's putting it mildly! (1-4)
I believe the answer is:
a-bomb
'an explosive' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for a-bomb that I've seen before include "Atomic weapon" , "Weapon of mass destruction" , "explosive" , "Killer" , "Nuclear weapon" .)