The most common and most likely answer to this crossword clue is the 5 plus 3 plus 6 letter word EARLY DAY MOTION
I've seen this crossword clue in The Times
E
A
R
L
Y
D
A
Y
M
O
T
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N
More clues leading to the result EARLY DAY MOTION
Buddy explains!
The straight part
We know that "not a matter for debate" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not a matter for debate = early day motion.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.