What you're nibbling on as you put the baby to sleep? (4,4)
I believe the answer is:
'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" .)