Notice given by authority on a piece of pasteboard (7)
I believe the answer is:
placard
'piece of pasteboard' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'notice given by authority on a' is the wordplay.
'notice' becomes 'card' (synonyms).
'given by' says to put letters next to each other.
'authority on' becomes 'pl' (I can't explain this - if you can you should believe this answer much more).
'pl'+'a'='pla'
'card' put after 'pla' is 'PLACARD'.
Can you help me to learn more?
(Other definitions for placard that I've seen before include "Poster" , "Sign posted in a public place as an advertisement" , "One taken on march" , "Make card, pal, into kind of ad" , "Printed notice" .)