Warning Brazilian city against diplomacy (4,3)
I believe the answer is:
riot act
'warning' is the definition.
(the Riot Act was read as a warning)
'brazilian city against diplomacy' is the wordplay.
'brazilian city' becomes 'Rio' (Rio de Janeiro).
'against' is a charade indicator (letters next to each other).
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'rio'+'tact'='RIOT ACT'
(Other definitions for riot act that I've seen before include "designed to nip trouble in the bud" , "It's read in the face of disturbances" , "Statute against rowdy behaviour" , "One read to the unruly once" , "something read to little devils?" .)