The most common and most likely answer to this crossword clue is the 9 letter word SYCAMORES
I've seen this crossword clue in The Guardian
S
Y
C
A
M
O
R
E
S
More clues leading to the result SYCAMORES
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 = sycamores.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 SYCAMORES mean?
SYCAMORES noun- Any of several North American plane trees, of the genus Platanus, especially Platanus occidentalis (American sycamore).
- A large British and European species of maple, Acer pseudoplatanus, known in North America as the sycamore maple.
- A large tree bearing edible fruit, Ficus sycomorus, allied to the common fig and found in Egypt and Syria; also called the sycamore fig or the fig-mulberry; the Biblical sycomore.