Brazilian diplomacy which prevents violence (4,3)
I believe the answer is:
riot act
'brazilian diplomacy which prevents violence' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'brazilian diplomacy' is the wordplay.
'brazilian' becomes 'rio' (I can't explain this - if you can you should give a lot more credence to this answer).
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'rio'+'tact'='RIOT ACT'
Can you help me to learn more?
(Other definitions for riot act that I've seen before include "something read to little devils?" , "Citator may read this as stern rebuke" , "designed to nip trouble in the bud" , "Old law designed to disperse a mob" , "It's read as a warning to disperse" .)