The most common and most likely answer to this crossword clue is the 9 letter word WATERLINE
I've seen this crossword clue in The Daily Telegraph
W
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Buddy explains!
The straight part
We know that "Mark in paper" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mark in paper = waterline.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 WATERLINE mean?
WATERLINE noun- A line formed by the surface of the water on the hull of a ship when she is afloat; any of a series of short lines marked on the hull to show where the waterline would be under different loadings.
- A horizontal line indicating the shape of an airfoil.
- A line showing where the water has been, usually a line separating dry land and wet areas; a watermark or tidemark.
- The inner rim of the eyelid, just behind the lash line; primarily used in reference to the application of eyeliner.