Cross about Beyeler's principal, not one framing Water Lilies (7)

I believe the answer is:

nelumbo

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'cross' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

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

Can you help me to learn more?

(Other definitions for nelumbo that I've seen before include "Water lily" , "Flower" , "plant" .)

I've seen this clue in The Telegraph.

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