To hell with cabbage salad! Pessimists believe it (4,3)
I believe the answer is:
sods law
'pessimists believe it' is the definition.
(belief that everything that can go wrong will)
'to hell with cabbage salad' is the wordplay.
'to hell with' becomes 'sod' (I can't justify this - if you can you should give a lot more credence to this answer).
'cabbage salad' becomes 'slaw'.
'sod'+'slaw'='SODS-LAW'
(Other definitions for sods law that I've seen before include "Theory that a dropped slice of bread will always land butter side down, for example" , "Why one can't win" , "Rule that if anything can go wrong it will" , "This is bound to happen" , "Expect the worst!" .)