Lacking oxygen, nuts contain astringent (6)
I believe the answer is:
tannic
'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" .)