A national concern under discussion (7,6)
I believe the answer is:
subject matter
'discussion' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'a national concern under' is the wordplay.
'a national' becomes 'subject' (synonyms).
'concern' becomes 'matter' (both can mean an affair or happening).
'under' means one lot of letters go next to another.
'subject'+'matter'='SUBJECT-MATTER'
Can you help me to learn more?
(Other definitions for subject matter that I've seen before include "Material at hand" , "The substance under consideration" , "Theme" , "citizen feel important" , "The topic being dealt with" .)