The most common and most likely answer to this crossword clue is the 8 letter word YEAR-END
Y
E
A
R
-
E
N
D
Buddy explains!

The straight part
We know that "31st December" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 31st december = year-end.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 YEAR-END mean?
YEAR-END adjective- On or happening at the end of the year (e.g., late December).