Why endlessly consume grain? (5)
I believe the answer is:
wheat
'grain?' is the definition.
(wheat is a type of cereal)
'why endlessly consume' is the wordplay.
'endlessly' means to remove the last letter.
'consume' becomes 'eat' (to consume something is to eat it).
'why' with its final letter removed is 'wh'.
'wh'+'eat'='WHEAT'
(Other definitions for wheat that I've seen before include "Cereal grass" , "Cereal grain" , "Cereal crop" , "(Pasta) cereal" , "Tall grass producing light brown grain" .)