Eco-friendly, but naive (5)
I believe the answer is:
green
I believe this clue is a double definition.
'eco-friendly' is the first definition.
(green can mean good for the environment)
'naive' is the second definition.
(similar in meaning)
'but' is the link.
(Other definitions for green that I've seen before include "Lincoln's colour?" , "Money" , "Environmental - colour" , "personification of envy" , "Unripe; envious" .)