Climate activist Thunberg (5)
I believe the answer is:
greta
'climate 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 "The familiar Ms Garbo, solitary film star" , "'I want to be alone' - ?.. Garbo (5)" , "Miss Garbo" , "Female" , "- Garbo, Swed. actress" .)