The most common and most likely answer to this crossword clue is the 3 plus 11 letter word AUF WIEDERSEHEN
I've seen this crossword clue in Financial Times
A
U
F
W
I
E
D
E
R
S
E
H
E
N
Buddy explains!
The straight part
We know that "goodbye" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case goodbye = auf wiedersehen.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.