Inexperienced environmentalist (5)
I believe the answer is:
green
This clue is a double definition.
'inexperienced' is the first definition.
(similar in meaning)
'environmentalist' is the second definition.
(member of the Green Party)
(Other definitions for green that I've seen before include "Environmentally sound" , "Ecologically beneficial" , "Caring for environment" , "Grassy enclosure, in a village say" , "Village space" .)