Flavouring ingredient essential to vegan's barley slices (6)

I believe the answer is:

garlic

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'flavouring' is the definition.
(I know that garlic is a type of flavouring)

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for garlic that I've seen before include "Bulb used in cooking" , "Pungent bulbs" , "Aioli ingredient" , "something with strong smell" , "Bulb used in cookery" .)

I've seen this clue in The Irish Times.

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