The most common and most likely answer to this crossword clue is the 9 letter word DOMINICAL
I've seen this crossword clue in The Daily Telegraph
D
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M
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More clues leading to the result DOMINICAL
Buddy explains!

The straight part
We know that "Sunday’s" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sunday’s = dominical.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 DOMINICAL mean?
DOMINICAL noun- Sunday.
- The Lord's Prayer.
- Of or pertaining to Jesus Christ as Lord.
- Pertaining to the Lord's Day, Sunday.
- Pertaining to the dominical letter, an ancient system for determining Sundays (particularly Easter Sunday) in any given year.