Ah! Trees swaying in May? (7)

I believe the answer is:

theresa

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But I can't explain it at all! Could you help me understand?

(Other definitions for theresa that I've seen before include "May's one" , "Woman" , "Maria -, Habsburg empress" , "Girl's name" , "Maria -, Austr. empress" .)

I've seen this clue in The Sun.

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