Range of mushrooms served up for tea from the bar menu (8)

I believe the answer is:

spectrum

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'range' is the definition.
(a spectrum is a range or variety)

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

(Other definitions for spectrum that I've seen before include "Light split by prism" , "All the bands of colour seen in a rainbow" , "The bands of colur as seen in rainbow" , "Broad range of related items" , "Wide range" .)

I've seen this clue in The Irish Times.

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