The most common and most likely answer to this crossword clue is the 5 letter word SENDS
I've seen this crossword clue in Financial Times
S
E
N
D
S
Buddy explains!

The straight part
We know that "posts" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case posts = sends.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 SENDS mean?
SENDS verb- To make something (such as an object or message) go from one place to another.
- To excite, delight, or thrill (someone).
- To bring to a certain condition.
- To dispatch an agent or messenger to convey a message, or to do an errand.
- To cause to be or to happen; to bestow; to inflict; to grant; sometimes followed by a dependent proposition.
- To pitch.
- To make a successful free ascent (i.e. not relying on gear) of a sport climbing route.