Reason things go wrong with potato salad? (7,3)
I believe the answer is:
murphys law
'reason things go wrong' is the definition.
('everything that can go wrong will')
'with potato salad?' is the wordplay.
'with' says to put letters next to each other.
'potato' becomes 'murphy' (synonyms).
'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 "Reason to expect the worst" , "Anything that can go wrong will go wrong" , "it implies bad result is likely" , "highly unpromising as a rule" , "Why things go wrong" .)