A growing reason to remember Ayrton (5)

I believe the answer is:

senna

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'a growing' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

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

Can you help me to learn more?

(Other definitions for senna that I've seen before include "given help to get in motion?" , "Natural laxative" , "Brazilian F1 champion, d. 1994" , "Champion driver.once" , "Formula One driver" .)

I've seen this clue in the Evening Standard.

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