Nuclear accord chief whip initially switched (4,3)
I believe the answer is:
test ban
'nuclear accord' is the definition.
The definition suggests a singular noun which matches the answer.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for test ban that I've seen before include "international bar" , "Agreement on limiting nuclear research" , "Prohibition on trying out atomic bombs" , "International agreement concerning nuclear weapons" , "Preclusion of H-bomb experimentation" .)