By God! Bohr and men designed nuclear weapon (8,4)
I believe the answer is:
hydrogen bomb
'nuclear weapon' is the definition.
(hydrogen bomb is a kind of nuclear weapon)
'by god bohr and men designed' is the wordplay.
'and' says to put letters next to each other.
'designed' indicates an anagram (designed with the letters).
'by'+'god'+'bohr'+'men'='bygodbohrmen'
'bygodbohrmen' anagrammed gives 'HYDROGEN-BOMB'.
(Other definitions for hydrogen bomb that I've seen before include "Nuke" , "Massively destructive explosive device" .)