Environmental activist Brockovich (4)
I believe the answer is:
erin
'environmental activist brockovich' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for erin that I've seen before include "Literary term for Ireland" , "'Come back to ...' (emigrant song)" , "Early and poetic name here" , "(Literary name for) Ireland" , "Poetically, Ireland" .)