Air letter? (9)
I believe the answer is:
character
I believe this clue is a double definition.
'air' is the first definition.
(air can mean a character or quality)
'letter?' is the second definition.
(written character)
(Other definitions for character that I've seen before include "I, for example" , "Nonconformist" , "Letter or number" , "Personal moral qualities" , "Someone in play" .)