Rehydrate food to cook what's on the menu (5,2,3,3)
I believe the answer is:
order of the day
'what's on the menu' is the definition.
'rehydrate food to cook' is the wordplay.
'to cook' indicates anagramming the letters.
'rehydrate'+'food'='rehydratefood'
'rehydratefood' with letters rearranged gives 'ORDER OF THE DAY'.
(Other definitions for order of the day that I've seen before include "Immediate business" , "Prevailing custom of the time" , "Something normal at a given time" , "Current business" , "Parliamentary agenda" .)