Fastening of the mushroom variety (6)

I believe the answer is:

button

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'fastening' is the definition.
(button is a kind of fastening)

I don't understand how the rest of the clue works.

(Other definitions for button that I've seen before include "bit of the mattress?" , "Bright as a - -" , "Electric switch operated by pressing" , "Knob or disc as clothes fastener" , "Fastener to fit through a hole" .)

I've seen this clue in the Evening Standard.

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