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 a hidden word (some of the 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" .)