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But if that is the case, I would expect “unusual” to be the penultimate word in the clue, so I’m confused now …

They were my original thoughts - that 'unusual' was superfluous or wronly placed in clue - hence Ithought I must be missing something ...

I don’t think you are, crates. The word inversion improves the surface but harms the wordplay. Better just to finish with “American verse, unusually”, imho. If unusual is required to indicate the rareness of the answer, it can’t qualify American.

Of course we may both be missing something!