The most common and most likely answer to this crossword clue is the 9 letter word POSTMARKS
I've seen this crossword clue in The Guardian
P
O
S
T
M
A
R
K
S
Buddy explains!
The straight part
We know that "letters must show these" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case letters must show these = postmarks.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 POSTMARKS mean?
POSTMARKS noun- A marking made by a postal service on a letter, package, postcard or the like, usually indicating the place where and the date and time when the item was received or processed for the first time, and often serving to cancel a postage stamp.
- To apply a postmark on.