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