The most common and most likely answer to this crossword clue is the 4 letter word MEME
I've seen this crossword clue in Independent
M
E
M
E
Buddy explains!
The straight part
We know that "online phenomenon" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case online phenomenon = meme .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 MEME mean?
MEME noun- Any unit of cultural information, such as a practice or idea, that is transmitted verbally or by repeated action from one mind to another in a comparable way to the transmission of genes.
- Something, usually humorous, which is copied and circulated online with slight adaptations, including quizzes, basic pictures, video templates etc.
- A myth circulating as truth; something ineffective presented as effective, or similar.
- To turn into a meme; to use a meme, especially to achieve something in real life.
- To create and use humorous memes.
- To joke around.