Conservationist's prefix (3)
I believe the answer is:
eco
'conservationist's prefix' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eco that I've seen before include "Umberto --, Italian writer" , "Concerned with the environment" , "Umberto -, Italian author (The Name of the Rose)" , "Environmental group" , "Green prefix" .)