Crossword Genius

Difficult to hide outside of the book (4,5)

Ross

I believe the answer is:

hard cover

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'outside of the book' is the definition.
I know nothing about this answer so I can't judge whether this works.

'difficult to hide' is the wordplay.
'difficult' becomes 'hard' (associated in meaning).
'to hide' becomes 'cover' (covering is a kind of hiding).
'hard'+'cover'='HARD COVER'

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I've seen this clue in the Sydney Morning Herald.
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