The most common and most likely answer to this crossword clue is the 2 plus 1 plus 4 plus 3 plus 5 letter word AT A LOSS FOR WORDS
I've seen this crossword clue in The Guardian
A
T
A
L
O
S
S
F
O
R
W
O
R
D
S
More clues leading to the result AT A LOSS FOR WORDS
Buddy explains!

The straight part
We know that "Stunned" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stunned = at a loss for words.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.