The most common and most likely answer to this crossword clue is the 13 letter word THUNDERSTRUCK
I've seen this crossword clue in The Times
T
H
U
N
D
E
R
S
T
R
U
C
K
More clues leading to the result THUNDERSTRUCK
- Amazed heavy vehicle moves noisily in front
- Astonished, inveighs against delivery vehicle
- Makes a loud noise with lorry being severely shaken
- Amazed as lorry speeds to overtake
- Amazed, seeing small pick-up possibly after crash
- Amazed to see second lorry after crash
- Thus nerd in crash with traffic staggered
- Business is booming at first having experienced shock
- So vehicle is broken into by crazy nerd? It makes one speechless
- Astounded by second pick-up following boom
Buddy explains!

The straight part
We know that "Astonished" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case astonished = thunderstruck.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 THUNDERSTRUCK mean?
THUNDERSTRUCK adjective- Astonished, amazed or so suddenly surprised as to be unable to speak.