Cracks joke child is about to comprehend (7)

I believe the answer is:

decodes

I'm a little stuck... Click here to teach me more about this clue!

'cracks' is the definition.
(I know this)

'joke child is about to comprehend' is the wordplay.
I cannot quite understand how this works, but
'joke' could be 'cod' (I've seen this before) and 'cod' is found within the answer.
The remaining letters 'dees' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Sunday Times.

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