Arrest leader of demonstration and member of Greens (7)
I believe the answer is:
Here is my best explanation:
'member of greens' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'arrest leader of demonstration' is the subsidiary indication.
'arrest' becomes 'collar' ('collar' can be a synonym of 'arrest').
'leader of' suggests taking the first letters.
The first letter of 'demonstration' is 'd'.
'and' is the link.
(Another definition for collard that I've seen is "Cabbage is the answer".)