The most common and most likely answer to this crossword clue is the 8 letter word POSTBAGS
P
O
S
T
B
A
G
S
More clues leading to the result POSTBAGS
Buddy explains!

The straight part
We know that "Mail carriers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mail carriers = postbags.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.
Crossword Answer definitions
POSTBAGS noun- A bag used for carrying post (mail)