Fungus in a bulb, light initially extinguished (6)

I believe the answer is:

agaric

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

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for agaric that I've seen before include "Type of mushroom" , "Fungus, eg, mushroom" , "fungal growth" , "Fungus of mushroom family" .)

I've seen this clue in The Guardian.

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