The most common and most likely answer to this crossword clue is the 10 letter word ASTONISHED
I've seen this crossword clue in The Sunday Telegraph
A
S
T
O
N
I
S
H
E
D
More clues leading to the result ASTONISHED
- Very surprised when hedonist collapses
- Nonplussed when fashion editor goes on rather!
- Full of amazement since a lot of weight I lost
- Surprised about name I dropped
- Blown away — and so is the organisation?
- Surprised since I dropped weight first
- Shocked by a dishonest organisation
- Dumbfounded by a dishonest arrangement
- Very surprised concerning name taken by one setter
- Staggered because heavy weight I dropped
Buddy explains!

The straight part
We know that "Staggered" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case staggered = astonished.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 ASTONISHED mean?
ASTONISHED verb- To surprise greatly.
- Amazed; surprised.