The most common and most likely answer to this crossword clue is the 9 letter word MAGNOLIAS
I've seen this crossword clue in Independent
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Buddy explains!
The straight part
We know that "trees" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case trees = magnolias.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 MAGNOLIAS mean?
MAGNOLIAS noun- A tree or shrub in any species of the genus Magnolia, many with large flowers and simple leaves.
- The flower of a magnolia tree.
- A native or resident of the American state of Mississippi.
- A creamy white colour, like that of some magnolia flowers.