Wife’s beginning with warm cereal (5)
I believe the answer is:
wheat
'cereal' is the definition.
(wheat is a kind of cereal)
'wife's beginning with warm' is the wordplay.
'beginning' indicates taking the first letters.
'with' is a charade indicator (letters next to each other).
'warm' becomes 'heat'.
The first letter of 'wifes' is 'w'.
'w'+'heat'='WHEAT'
(Other definitions for wheat that I've seen before include "Bread ingredient" , "Tall grass producing light brown grain" , "cereal's required" , "Type of cereal" , "staple food" .)