The most common and most likely answer to this crossword clue is the 8 letter word AEROGRAM
I've seen this crossword clue in Financial Times
A
E
R
O
G
R
A
M
More clues leading to the result AEROGRAM
Buddy explains!

The straight part
We know that "Letter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case letter = aerogram.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 AEROGRAM mean?
AEROGRAM noun- A wireless message.
- A telegram whose transmission included at least one segment sent via airplane.
- A thin piece of foldable and gummed paper for writing a letter and serving as its own envelope for transit via airmail.
- A pneumogram.