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Lacking oxygen, nuts contain astringent (6)

Ross

I believe the answer is:

tannic

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'lacking' is the definition.
Although both the answer and definition are adjectives, I don't see how they can define each other.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for tannic that I've seen before include "This acid is used for dyeing and leather-making" , "& 14 Across Chemical in e.g. tea" , "Related to compound found in tea, used for dyeing" , "Acid in tea and wine" , "Relating to a substance in tea and wine" .)

I've seen this clue in The Sunday Times and The Times.
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