Smooth pistachio nuts eaten by horse? All but the middle (13)

I believe the answer is:

sophisticated

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

(Other definitions for sophisticated that I've seen before include "Cosmopolitan" , "Cultured; refined" , "Possessed of culture, refinement and 24" , "Cultured; elegant" , "Urbane" .)

I've seen this clue in The Telegraph.

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