The stuff of legend: from day one being seen in a sweater! (9)

I believe the answer is:

arthurian

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

(Other definitions for arthurian that I've seen before include "Of a legendary king" , "Relating to the once and future king" , "Sort of legend" , "of legendary hero" , "Like legends" .)

I've seen this clue in The Times.

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