Expert activist in the arts? (4)

I believe the answer is:

star

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

'expert' is the definition.
(star is a kind of expert)

'activist in the arts?' is the wordplay.
'arts' anagrams to 'STAR'.
Though, I am not sure how this is indicated.

Can you help me to learn more?

(Other definitions for star that I've seen before include "Top celebrity" , "Public figure" , "One guiding" , "Celebrity - heavenly body" , "Take the lead role" .)

I've seen this clue in the Evening Standard.

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