The most common and most likely answer to this crossword clue is the 10 letter word PREDICTION
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Buddy explains!

The straight part
We know that "Announcement of what’s to come" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case announcement of what’s to come = prediction.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 PREDICTION mean?
PREDICTION noun- A statement of what will happen in the future.
- A probability estimation based on statistical methods.