The most common and most likely answer to this crossword clue is the 5 plus 6 letter word CHAIN LETTER
C
H
A
I
N
L
E
T
T
E
R
More clues leading to the result CHAIN LETTER
Buddy explains!
The straight part
We know that "What may arrive in the post" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what may arrive in the post = chain letter.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.