Overtake one on river, blooming thing (7-6)

I believe the answer is:

passion-flower

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

'overtake one on river' is the wordplay.
'overtake' becomes 'pass' (as in passing on a road).
'one' becomes 'i' (Roman numeral).
'river' becomes 'flower' (a river flows).
'pass'+'i'+'on'+'flower'='PASSION-FLOWER'

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I've seen this clue in The Times.

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