The most common and most likely answer to this crossword clue is the 5 letter word WOWEE
W
O
W
E
E
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! = wowee.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 WOWEE mean?
WOWEE interjection- Wow; expressing astonishment, surprise or excitement