The most common and most likely answer to this crossword clue is the 11 letter word YLANG-YLANG
I've seen this crossword clue in The Guardian
Y
L
A
N
G
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Y
L
A
N
G
More clues leading to the result YLANG-YLANG
Buddy explains!

The straight part
We know that "Tree" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tree = ylang-ylang.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 YLANG-YLANG mean?
YLANG-YLANG noun- The fragrant flower of a tropical tree (Cananga odorata) native to southeast Asia.
- An essential oil derived from this flower used in aromatherapy and perfumes.
- The tree on which ylang ylang flowers grow.