Crossword Genius

What you're nibbling on as you put the baby to sleep? (4,4)


I believe the answer is:

rock cake

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'sleep?' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for rock cake that I've seen before include "Baked tooth-breaker?" , "Rough-looking hard bun" .)

I've seen this clue in the Evening Standard.
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