Food products sorted at the postal centre (4)

I believe the answer is:

oats

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

I don't understand the rest of the clue.

(Other definitions for oats that I've seen before include "Grain for porridge" , "Cereal plants" , "Cereal food for horses" , "Cereal for porridge" , "Seed grasses" .)

I've seen this clue in the Evening Standard.

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