Cases to be cracked over breakfast? (9)
I believe the answer is:
eggshells
'cases' is the definition.
(cases for eggs)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for eggshells that I've seen before include "See 1" , "They're hard to hold yolks" , "Walk delicately as if on these" , "Tip-toe gently like walking on ........." .)