Rising at first, but getting smaller overall (7)
I believe the answer is:
warning
'rising at first' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe you can see a link between them that I don't see?
I can't explain the remainder of the clue.
(Other definitions for warning that I've seen before include "Message informing of danger" , "Previous notice" , "Cautionary advice" , "Caution - threat" , "Advance notice" .)