In hairdressing, the best union (7)

I believe the answer is:

topknot

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

(Other definitions for topknot that I've seen before include "Hair extension" , "Decorative bow on head" , "Crest of hair or feathers on the crown of the head" , "Crest - tuft of hair" , "Tuft or crest on crown of head" .)

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