The most common and most likely answer to this crossword clue is the 13 letter word BIODEGRADABLE
I've seen this crossword clue in The Times
B
I
O
D
E
G
R
A
D
A
B
L
E
Buddy explains!

The straight part
We know that "Eco-friendly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case eco-friendly = biodegradable.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 BIODEGRADABLE mean?
BIODEGRADABLE noun- Any material that can be decomposed by biological activity.
- Capable of being decomposed by biological activity, especially by microorganisms.