The most common and most likely answer to this crossword clue is the 8 letter word RAREFIED
I've seen this crossword clue in Independent
R
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More clues leading to the result RAREFIED
Buddy explains!

The straight part
We know that "high" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case high = rarefied.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 RAREFIED mean?
RAREFIED adjective- Distant from the lives and everyday concerns of ordinary people; esoteric, exclusive, select.
- Elevated in style or nature, sublime; of high intellectual or moral value.
- (of a gas etc.) Less dense than usual; thin.
- To make rare, thin, porous, or less dense
- To expand or enlarge without adding any new portion of matter to.