Artistic freedom — on Celtic piece (anag) (6,7)

I believe the answer is:

poetic licence

(Other definitions for poetic licence that I've seen before include "Bending the rules for artistic effect" , "Departing from accuracy for artistic effect" , "Creative freedom for writer or artist" , "Excuse for rule-breaking" , "A taking of liberties" .)

I've seen this clue in The Guardian.

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