The most common and most likely answer to this crossword clue is the 5 plus 4 letter word THEME SONG
I've seen this crossword clue in The Guardian
T
H
E
M
E
S
O
N
G
More clues leading to the result THEME SONG
- That elementary particle finally revealing signature in air
- Signature tune: The Dancing Gnomes
- Suicide is Painless? No – gets me upset outside hospital
- Seem upset wearing G-string for such a melody
- Air show has them for one hugging boy
- The One Show tune, perhaps
- Track particle drunk by the gallon
- Half of them, essentially best in band compose the signature tune
- Show’s intro gets me mostly hot ‘n’ bothered
Buddy explains!
The straight part
We know that "signature in air" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case signature in air = theme song.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.