Ruminant animals lacking good cereal grass (4)

I believe the answer is:

oats

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'grass' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for oats that I've seen before include "Porridge ingredients" , "Hardy cereals eaten by horses" , "Young folk might sow wild ones" , "Cereal grass and seeds" , "Basis of porridge" .)

I've seen this clue in The Times.

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