The most common and most likely answer to this crossword clue is the 10 letter word SKYWRITING
I've seen this crossword clue in The Daily Telegraph
S
K
Y
W
R
I
T
I
N
G
Buddy explains!

The straight part
We know that "letters on air" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case letters on air = skywriting.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 SKYWRITING mean?
SKYWRITING noun- Messages, left by leaving a trail of smoke from an airplane, that are visible from the ground.