Why things go wrong with potato salad? (7,3)
I believe the answer is:
murphys law
'why things go wrong' is the definition.
(I've seen this before)
'with potato salad?' is the wordplay.
'with' is a charade indicator (letters next to each other).
'potato' becomes 'murphy' ('murphy' can be a synonym of 'potato').
'salad?' becomes 'slaw' (slaw is a kind of salad).
'murphy'+'slaw' is 'MURPHYS LAW'.
(Other definitions for murphys law that I've seen before include "Anything that can go wrong will go wrong" , "it implies bad result is likely" , "Reason to expect the worst" , "highly unpromising as a rule" , "Why things go wrong" .)