Brazilian diplomacy is a check on disorder (4,3)
I believe the answer is:
riot act
'brazilian diplomacy' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'is a check on disorder' is the wordplay.
'is a check' becomes 'act' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on' says to put letters next to each other.
'disorder' becomes 'riot' (riot is a kind of disorder).
'act' after 'riot' is 'RIOT ACT'.
Can you help me to learn more?
(Other definitions for riot act that I've seen before include "measure to keep order" , "designed to nip trouble in the bud" , "legislation delivered to the mob" , "what constitutes warning" , "something read to little devils?" .)