The most common and most likely answer to this crossword clue is the 6 letter word E-CARD
E
-
C
A
R
D
Buddy explains!
The straight part
We know that "online message" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case online message = e-card.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 E-CARD mean?
E-CARD noun- A computerized greeting card, typically hosted on a Web site to which the recipient is directed by an e-mail message.