The most common and most likely answer to this crossword clue is the 4 letter word VLOG
I've seen this crossword clue in The Guardian
V
L
O
G
Buddy explains!

The straight part
We know that "posts online" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case posts online = vlog.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 VLOG mean?
VLOG noun- A weblog using video as its primary presentation format.
- To contribute to a video weblog.
- To post (something) to a video weblog.