Why things go wrong in ground cabbage dish (4,3)
I believe the answer is:
sods law
'why things go wrong' is the definition.
(humorous law saying the worst outcome will happen)
'ground cabbage dish' is the wordplay.
'ground' becomes 'sod' (both can mean a piece of earth).
'cabbage dish' becomes 'slaw' (dish made from cabbage).
'sod'+'slaw'='SODS-LAW'
'in' acts as a link.
(Other definitions for sods law that I've seen before include "That which suggests that, if something can go wrong, it will!" , "this means I'm bound to get some!" , "Pessimistic theory" , "Rule that if anything can go wrong it will" , "Why one can't win" .)