The most common and most likely answer to this crossword clue is the 8 letter word OBLIGING
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Buddy explains!
The straight part
We know that "courteous" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case courteous = obliging.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 OBLIGING mean?
OBLIGING verb- To constrain someone by force or by social, moral or legal means.
- To do (someone) a service or favour (hence, originally, creating an obligation).
- To be indebted to someone.
- The imposition of an obligation.
- Happy and ready to do favours for others.