The most common and most likely answer to this crossword clue is the 6 letter word EOLIAN
I've seen this crossword clue in The Times
E
O
L
I
A
N
More clues leading to the result EOLIAN
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The straight part
We know that "by air" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case by air = eolian.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 EOLIAN mean?
EOLIAN adjective- Of, or relating to the wind.
- Carried, deposited or eroded by the wind.
- Aeolian mode