The most common and most likely answer to this crossword clue is the 2 plus 8 plus 3 letter word ST STEPHENS DAY
I've seen this crossword clue in The Times
S
T
S
T
E
P
H
E
N
S
D
A
Y
Buddy explains!
The straight part
We know that "it won't arrive until after Christmas" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it won't arrive until after christmas = st stephens day.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.