The most common and most likely answer to this crossword clue is the 4 plus 3 plus 3 plus 3 letter word RING OUT THE OLD
I've seen this crossword clue in The Daily Telegraph
R
I
N
G
O
U
T
T
H
E
O
L
D
Buddy explains!
The straight part
We know that "end-of-year message" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case end-of-year message = ring out the old.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.