Crossword Genius

Could apple be so thoroughly corrupt? (6,2,3,4)

Ross

I believe the answer is:

rotten to the core

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

'could apple be so thoroughly' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'corrupt?' is the wordplay.
I cannot quite understand how this works, but
'corrupt?' could be 'rotten' ('rotten' can be a synonym of 'corrupt') and 'rotten' is found in the answer.
The remaining letters 'tothecore' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

(Another definition for rotten to the core that I've seen is " A bad apple".)

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