The most common and most likely answer to this crossword clue is the 7 plus 9 letter word ETERNAL (ALTERNE)
E
T
E
R
N
A
L
(
A
L
T
E
R
N
E
)
Buddy explains!
The straight part
We know that "plant community different from its neighbour" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plant community different from its neighbour = eternal (alterne).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.