Prefix with friendly (3)
I believe the answer is:
eco
'prefix with friendly' 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 author (The Name of the Rose)" , "Concerned with the environment" , "Green prefix" , "Umberto --, Italian writer" , "Umberto - - , novelist" .)