Some nuclear material, as a weapon (3)
I believe the answer is:
arm
'a weapon' is the definition.
('arm' can be a synonym of 'weapon')
'some nuclear material as' is the wordplay.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'ARM' is hidden within 'nuclear material as'.
(Other definitions for arm that I've seen before include "Shoulder to fingertip" , "Equip with weapons" , "Ram into limb" , "Sleeve" , "Limb or branch" .)