Activist Thunberg (5)
I believe the answer is:
greta
'activist thunberg' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for greta that I've seen before include ".....Garbo, who wanted to be alone" , "lady" , "Female" , "The familiar Ms Garbo, solitary film star" , "Miss Garbo" .)