So cereals must be mixed in dish (9)
I believe the answer is:
casserole
'dish' is the definition.
(I know that casserole is a type of dish)
'so cereals must be mixed' is the wordplay.
'must be mixed' indicates anagramming the letters.
'so'+'cereals'='socereals'
'socereals' with letters rearranged gives 'CASSEROLE'.
'in' is the link.
(Other definitions for casserole that I've seen before include "Stew-dish" , "Make a stew" , "(Food cooked in a) stewpot" , "Large cooking dish" , "Stew; dish" .)