The most common and most likely answer to this crossword clue is the 7 letter word GREENIE
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More clues leading to the result GREENIE
Buddy explains!

The straight part
We know that "Environmentalist down under" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case environmentalist down under = greenie.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does GREENIE mean?
GREENIE noun- An environmentalist; someone who shows concern for the environment.
- (by extension) A member of the Green Party.
- (Wyoming) A person from Colorado; after the color of the Colorado license plate.
- An unripe fruit.
- A beginner, a novice.
- A small, green object or creature.