The most common and most likely answer to this crossword clue is the 8 letter word GREENEST
I've seen this crossword clue in The Guardian
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Buddy explains!
The straight part
We know that "most eco-friendly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most eco-friendly = greenest.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 GREENEST mean?
GREENEST adjective- Having green as its color.
- (of people) Sickly, unwell.
- Unripe, said of certain fruits that change color when they ripen.
- (of people) Inexperienced.
- (of people) Naive or unaware of obvious facts.
- (of people) Overcome with envy.
- Environmentally friendly.
- Describing a pitch which, even if there is no visible grass, still contains a significant amount of moisture.
- Of bacon or similar smallgoods: unprocessed, raw, unsmoked; not smoked or spiced.
- Not fully roasted; half raw.
- Of freshly cut wood or lumber that has not been dried: containing moisture and therefore relatively more flexible or springy.
- High or too high in acidity.
- Full of life and vigour; fresh and vigorous; new; recent.
- Having a sexual connotation.
- Having a color charge of green.
- Being or relating to the green currencies of the European Union.