The most common and most likely answer to this crossword clue is the 10 letter word CIRCUMFLEX
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More clues leading to the result CIRCUMFLEX
Buddy explains!

The straight part
We know that "Mark" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mark = circumflex.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 CIRCUMFLEX mean?
CIRCUMFLEX noun- A diacritical mark (ˆ) placed over a vowel in the orthography or transliteration of many languages to change its pronunciation; while in some other languages over a consonant.
- To mark or pronounce with a circumflex.
- Having a circumflex mark.
- Curving around.