Green manure covers hedges (3)
I believe the answer is:
eco
'green' is the definition.
(eco-friendly)
'manure covers hedges' is the wordplay.
'hedges' indicates the answer is hidden within the clue (hedge can mean enclose).
'ECO' is hidden within 'manure covers'.
(Other definitions for eco that I've seen before include "Umberto - - , novelist" , "Concerned with the environment" , "Green prefix" , "Umberto -, Italian author (The Name of the Rose)" , "Umberto --, Italian writer" .)