Immature environmentalist (5)
I believe the answer is:
green
This clue is a double definition.
'immature' 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 "17's colour" , "Like lettuce" , "conservationist" , "Village space" , "The colour of immaturity" .)