Crossword Genius

Everlasting rock lyric he hums (11)

Ross

I believe the answer is:

helichrysum

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'everlasting' is the definition.
The definition and answer can be both plants as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

I don't understand the remainder of the clue.

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I've seen this clue in The Sunday Times and The Times.
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