The most common and most likely answer to this crossword clue is the 9 letter word THROWAWAY
I've seen this crossword clue in The Guardian
T
H
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W
A
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Buddy explains!

The straight part
We know that "Such a casual remark" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case such a casual remark = throwaway.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 THROWAWAY mean?
THROWAWAY noun- Something temporary and disposable.
- Disposable; intended for a single use prior to being discarded.
- Extemporaneous; off the cuff.
- Selected or used without care or attention.