Subject of treaty dubious at best, note (4,3)
I believe the answer is:
test ban
'subject of treaty' is the definition.
The definition suggests a singular noun which matches the answer.
'dubious at best note' is the wordplay.
'dubious' indicates anagramming the letters.
'note' becomes 'n' (abbreviation).
'at'+'best'='atbest'
'atbest' anagrammed gives 'testba'.
'testba'+'n'='TEST BAN'
Can you help me to learn more?
(Other definitions for test ban that I've seen before include "Agreement on limiting nuclear research" , "Preclusion of H-bomb experimentation" , "Prohibition on trying out atomic bombs" , "international agreement on arms" , "Check on risky race" .)