The most common and most likely answer to this crossword clue is the 7 letter word AIRMAIL
I've seen this crossword clue in Financial Times
A
I
R
M
A
I
L
More clues leading to the result AIRMAIL
- Letters from within Cairo and Lima, possibly
- Letters brought over in crates?
- Postal system in trouble after beginning to reach breaking point
- Trouble besetting Irish scholar in old-fashioned communication
- Perfect way to travel, securing minister’s first international post
- To be sick confines girl to such correspondence
- Two things Charles is but Camilla isn’t, in speech to deliver abroad?
- Flying service is perfect, bar getting millions in
- Strain with money trouble in post overseas
- Put on the radio, say, man on the radio offers news from overseas
Buddy explains!

The straight part
We know that "Letters brought over in crates" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case letters brought over in crates = airmail.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 AIRMAIL mean?
AIRMAIL noun- The system of conveying mail using aircraft.
- The items of mail so carried.
- To send mail by air.
- To (unintentionally) throw the ball well over a fielder's head where that fielder is unable to make a play on the ball.