The most common and most likely answer to this crossword clue is the 9 letter word AMAZINGLY
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More clues leading to the result AMAZINGLY
Buddy explains!

The straight part
We know that "causing astonishment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case causing astonishment = amazingly.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 AMAZINGLY mean?
AMAZINGLY adverb- In an amazing manner; in a way that causes amazement; wonderfully.
- Difficult to believe; strange but true.
- To a wonder-inspiring extent.