Fungus in wrinkle beneath chestnut's base (7)

I believe the answer is:

truffle

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'fungus' is the definition.
(I know that truffle is a type of fungus)

I don't understand the remainder of the clue.

(Other definitions for truffle that I've seen before include "Earthnut" , "Sweet - edible fungus growing underground" , "2 (anag)" , "Edible fungus -- chocolate sweet" , "Fungoid delicacy" .)

I've seen this clue in The Guardian.

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