Rising at first, but getting smaller overall (7)
I believe the answer is:
Here is my best explanation:
'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", "Cautionary advice", "caution", "Previous notice", "Caution - threat", "Advance notice" and "Tip-off".)