An eccentric shell? (3-4)

I believe the answer is:

nut-case

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

'an eccentric' is the definition.

'shell?' is the wordplay.
I cannot really see how this works, but
'shell?' could be 'case' ('case' can be a synonym of 'shell') and 'case' is found in the answer.
The remaining letters 'nut' is a valid word which might be clued in a way I don't see.
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 Sydney Morning Herald.

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