Healthy food for environmentalists (6)
I believe the answer is:
greens
I believe this clue is a double definition.
'healthy food' is the first definition.
(informal term for vegetables)
'environmentalists' is the second definition.
(members of the Green Party)
'for' acts as a link.
(Other definitions for greens that I've seen before include "Leaves" , "Colours" , "those joining in the party" , "Putting areas" , "Political party" .)