The most common and most likely answer to this crossword clue is the 11 letter word SCAREMONGER
I've seen this crossword clue in The Daily Telegraph
S
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More clues leading to the result SCAREMONGER
Buddy explains!

The straight part
We know that "Awful screamer keeping on, originator of gloom" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case awful screamer keeping on, originator of gloom = scaremonger.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SCAREMONGER mean?
SCAREMONGER noun- Someone who spreads worrying rumours or needlessly alarms people.
- To spread worrying rumours.