Brazilian diplomacy to be applied where there's trouble? (4,3)
I believe the answer is:
riot act
'to be applied where there's trouble?' is the definition.
'brazilian diplomacy' is the wordplay.
'brazilian' becomes 'rio' (I can't explain this - if you can you should believe this answer much more).
'diplomacy' becomes 'tact' (I've seen this before).
'rio'+'tact'='RIOT ACT'
(Other definitions for riot act that I've seen before include "used to deal with unrest" , "designed to nip trouble in the bud" , "It's read as a warning to disperse" , "One read to the unruly once" , "This is read for a severe warning" .)