Legendary Celt in groundbreaking art: hurrah! (4,6)

I believe the answer is:

king arthur

I'm a little stuck... Click here to teach me more about this clue!

'legendary celt' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'in groundbreaking art hurrah' is the wordplay.
'in' indicates a hidden word.
'KING ARTHUR' is hidden in the letters of 'groundbreaking art hurrah'.

Can you help me to learn more?

(Other definitions for king arthur that I've seen before include "Legendary monarch" , "Guinevere's husband" .)

I've seen this clue in The Everyman puzzle in The Observer.

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