Humiliation essentially stops Warren becoming a grass (5)

I believe the answer is:

maize

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'grass' is the definition.
(I know that maize is a type of cereal)

I don't understand the rest of the clue.

(Other definitions for maize that I've seen before include "source of food" , "Source of oil" , "plant with ears" , "one could make a meal of it" , "Tall cereal plant bearing grain on large pods" .)

I've seen this clue in The Independent.

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