The most common and most likely answer to this crossword clue is the 3 plus 4 letter word RED CRAB
I've seen this crossword clue in Financial Times
R
E
D
C
R
A
B
More clues leading to the result RED CRAB
Buddy explains!
The straight part
We know that "population all over Christmas" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case population all over christmas = red crab.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.