Hearts of the vain yakuza captured in poetry (5)

I believe the answer is:

haiku

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'poetry' is the definition.
(I know that haiku is a type of poem)

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

(Other definitions for haiku that I've seen before include "17-syllable Japanese verse form" , "Japanese three-line verse form" , "Stylised Japanese poem of 17 syllables" , "Japanese three-line poem of 17 syllables" , "Type of Japanese poem" .)

I've seen this clue in The Everyman puzzle in The Observer.

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