The most common and most likely answer to this crossword clue is the 3 plus 6 letter word PEN FRIEND
I've seen this crossword clue in The Daily Telegraph
P
E
N
F
R
I
E
N
D
More clues leading to the result PEN FRIEND
Buddy explains!
The straight part
We know that "Poster" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case poster = pen friend.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 PEN FRIEND mean?
PEN FRIEND noun- A person, often in a foreign country, with whom one exchanges friendly correspondence but whom one never or rarely meets.