Not the only one in flower (5)

I believe the answer is:

petal

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'flower' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

(Other definitions for petal that I've seen before include "Coloured segment of flower" , "Bit of blooming" , "Floral segment" , "Floral feature" , "Flowery term of endearment?" .)

I've seen this clue in the Evening Standard.

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