Pinter's style evolves constantly (12)

I believe the answer is:

persistently

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'constantly' is the definition.
Both the definition and answer are adverbs. Maybe they are linked in a way I don't understand?

'pinter's style evolves' is the wordplay.
'pinter's style' anagrams to 'PERSISTENTLY'.
But, I am not clear how the anagram is indicated.

Can you help me to learn more?

(Other definitions for persistently that I've seen before include "Continually" , "Doggedly carrying on" , "In a tenacious manner" , "Incessantly; resolutely" .)

I've seen this clue in The Telegraph.

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