Crossword Genius

A fair night's rest? (3,5,2,3,4)

Ross

I believe the answer is:

the sleep of the just

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'a fair night's rest?' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'fair night's rest?' is the wordplay.
I cannot quite see how this works, but
'fair' could be 'just' (synonyms) and 'just' is found in the answer.
'rest?' could be 'sleep' ('sleep' can be a synonym of 'rest') and 'sleep' is found within the answer.
This explanation may well be incorrect...

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I've seen this clue in the Evening Standard.
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