The most common and most likely answer to this crossword clue is the 5 letter word MACHE
I've seen this crossword clue in The Times
M
A
C
H
E
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The straight part
We know that "hard paper" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hard paper = mache.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 MACHE mean?
MACHE noun- Valerianella locusta, a small dicot annual plant of the family Caprifoliaceae often cultivated as a salad green or herb.
- A former unit of volumic radioactivity: the quantity of radon (ignoring its daughters) per litre of air which ionizes a sustained current of 0.001 esu.
- Glutinous rice balls flavoured with coconut and pandan.