The most common and most likely answer to this crossword clue is the 6 letter word WONDER
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More clues leading to the result WONDER
Buddy explains!

The straight part
We know that "That’s amazing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that’s amazing = wonder.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 WONDER mean?
WONDER noun- Something that causes amazement or awe; a marvel.
- Something astonishing and seemingly inexplicable.
- Someone very talented at something, a genius.
- The sense or emotion which can be inspired by something curious or unknown; surprise; astonishment, often with awe or reverence.
- A mental pondering, a thought.
- A kind of donut; a cruller.
- To be affected with surprise or admiration; to be struck with astonishment; to be amazed; to marvel; often followed by at.
- To ponder; to feel doubt and curiosity; to query in the mind.