The most common and most likely answer to this crossword clue is the 4 letter word DEMY
D
E
M
Y
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The straight part
We know that "Writing paper" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case writing paper = demy.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 DEMY mean?
DEMY noun- A printing paper size, 17½ inches by 22½ inches.
- One holding a demyship, a kind of scholarship for Magdalen College, Oxford.
- Junior scholar, specifically at Magdalen College, Oxford.