The most common and most likely answer to this crossword clue is the 3 plus 3 letter word YOU DON
Y
O
U
D
O
N
More clues leading to the result YOU DON
Buddy explains!
The straight part
We know that "‘How astonishing/one thinks" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ‘how astonishing/one thinks = you don.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.