The most common and most likely answer to this crossword clue is the 7 letter word POSTBAG
I've seen this crossword clue in The Guardian
P
O
S
T
B
A
G
More clues leading to the result POSTBAG
- After capture, letters received
- Office chat about the letters
- Letters received ahead of good job taken with airline
- Letters seeing job secure?
- Job secure? Sack given peripatetic worker
- Recalled gossip after office received letters
- Letters received after getting sack
- The answer contains a mixture of letters
- Speciality following appointment shown in letters
- Many letters received after a number of animals shot
Buddy explains!

The straight part
We know that "the letters" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the letters = postbag.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 POSTBAG mean?
POSTBAG noun- A bag used for carrying post (mail)