Common environmentalist (5)
I believe the answer is:
green
This clue is a double definition.
'common' is the first definition.
(synonyms)
'environmentalist' is the second definition.
(member of the Green Party)
(Other definitions for green that I've seen before include "right environmentally" , "Common, inexperienced" , "The colour of growing grass" , "The colour of immaturity" , "part of the golf course" .)