The most common and most likely answer to this crossword clue is the 7 letter word BYE-BYE
I've seen this crossword clue in The Guardian
B
Y
E
-
B
Y
E
Buddy explains!
The straight part
We know that "so long" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case so long = bye-bye.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 BYE-BYE mean?
BYE-BYE noun- A goodbye.
- Bedtime for a toddler, going to sleep, going to bed.
- Goodbye.