Ingestible powder in magazine seized by bookmaker (8)

I believe the answer is:

cornmeal

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'bookmaker' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for cornmeal that I've seen before include "Food" , "Cook could use such" .)

I've seen this clue in The Independent.

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