Nuclear treaty subject (4,3)
I believe the answer is:
test ban
'nuclear treaty subject' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for test ban that I've seen before include "Prohibition on trying out atomic bombs" , "Preclusion of H-bomb experimentation" , "international agreement on arms" , "Battens (anag.)" , "ignorant students might favour?" .)