Activist Brockovich (4)
I believe the answer is:
erin
'activist brockovich' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for erin that I've seen before include "Poetic name for Ireland" , "Another name for 11 ac" , "Literary term for Ireland" , "Old Ireland" , "Romantic, archaic name for Ireland" .)