The most common and most likely answer to this crossword clue is the 8 plus 5 letter word BUSINESS CLASS
I've seen this crossword clue in The Sunday Telegraph
B
U
S
I
N
E
S
S
C
L
A
S
S
More clues leading to the result BUSINESS CLASS
Buddy explains!

The straight part
We know that "where some high-fliers sit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case where some high-fliers sit = business class.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.