The most common and most likely answer to this crossword clue is the 10 letter word ACROPHOBIC
I've seen this crossword clue in The Times
A
C
R
O
P
H
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B
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C
Buddy explains!

The straight part
We know that "suffering at high levels" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case suffering at high levels = acrophobic.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.
Crossword Answer definitions
ACROPHOBIC noun- A person who has acrophobia.
- Of or pertaining to or suffering from acrophobia