The first one was dropped on Hiroshima (4,4)
I believe the answer is:
atom bomb
'the first one was dropped on hiroshima' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for atom bomb that I've seen before include "Nuclear device" , "The first one was dropped on Hiroshima" , "Explosive device" , "Big weapon" , "Nuclear weapon" .)