The most common and most likely answer to this crossword clue is the 8 letter word DIERESIS
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More clues leading to the result DIERESIS
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 = dieresis.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 DIERESIS mean?
DIERESIS noun- A diacritic ( ¨ ) placed over a vowel letter (especially the second of two consecutive ones) indicating that it is sounded separately, usually forming a distinct syllable, as in the English words naïve, Noël and Brontë, the French haïr and the Dutch ruïne.
- Distraction; the separation of a vowel, often a diphthong, into two distinct syllables.
- A natural break in rhythm when a word ends at the end of a metrical foot, in a line of verse.
- Hiatus; the occurrence of separate vowel sounds in adjacent syllables without an intervening consonant.