The most common and most likely answer to this crossword clue is the 10 letter word TROPOPAUSE
I've seen this crossword clue in The Guardian
T
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O
P
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Buddy explains!
The straight part
We know that "It’s up in the air /It’s up in the air" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s up in the air /it’s up in the air = tropopause.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 TROPOPAUSE mean?
TROPOPAUSE noun- The zone of transition between the troposphere and the stratosphere (approximately 13 kilometers). The tropopause normally occurs at an altitude of between 25,000 and 45,000 feet in polar and temperate zones. It occurs at 55,000 feet in the tropics.