Having cracked his nut, is prevented from leaving (4,2)

I believe the answer is:

shut in

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'having cracked his nut is prevented from leaving' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.

'having cracked his nut is prevented from leaving' is the wordplay.
'his nut' anagrams to 'SHUT-IN'.
Though, I'm not sure how this is indicated.

Can you help me to learn more?

(Another definition for shut in that I've seen is " Enclosed".)

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