Spirit of writer transformed internally (4)

I believe the answer is:

brio

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'spirit' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

(Other definitions for brio that I've seen before include "Dash or verve, from Italian" , "Spiritedness" , "Enthusiasm" , "'Liveliness, vivacity (4)'" , "Vigour (in music)" .)

I've seen this clue in The Telegraph.

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