Cases one's crackers to tackle (9)

I believe the answer is:

nutshells

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

'one's crackers to tackle' is the wordplay.
'one's' becomes 'shell' (I can't explain this - if you can you should believe this answer much more).
'crackers' becomes 'nuts' (synonyms).
'to tackle' is an insertion indicator (tackle can mean to take hold of).
'shell' going within 'nuts' is 'NUTSHELLS'.

Can you help me to learn more?

I've seen this clue in The Times.

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