Clue: This is how daughter prepared cabbage: why does it all go wrong? (4,3)
Answer: SOD’S LAW
Explanation: Definition: why does it all go wrong?
A charade of SO ('this is how') plus D ('daughter') plus SLAW ('prepared cabbage'). Murphy's law: if it can go wrong, it will; sod's law: if it can go wrong, it has.