Possible Clues:

• Case where head's agony is revealed
• Casing for kernel
• Shut Nell out, in case
• Summary holder?
• Hazel's external protection - naturally
• One may get cracked, mad - a terrible situation
• It holds the kernel (or summary?)
• Hard carapace around small fruits
• It's hard, containing a possible source of allergy
• Fool' s given very hard time in this, to be succinct