The most common and most likely answer to this crossword clue is the 2 plus 1 plus 5 plus 4 letter word AT A LATER DATE
I've seen this crossword clue in The Guardian
A
T
A
L
A
T
E
R
D
A
T
E
Buddy explains!
The straight part
We know that "in due course" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in due course = at a later date.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.