The most common and most likely answer to this crossword clue is the 13 letter word ENVIRONMENTAL
I've seen this crossword clue in The Guardian
E
N
V
I
R
O
N
M
E
N
T
A
L
More clues leading to the result ENVIRONMENTAL
Buddy explains!

The straight part
We know that "green" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case green = environmental.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 ENVIRONMENTAL mean?
ENVIRONMENTAL adjective- Pertaining to the environment.