The most common and most likely answer to this crossword clue is the 9 letter word HORNBEAMS
I've seen this crossword clue in The Times
H
O
R
N
B
E
A
M
S
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 = hornbeams.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 HORNBEAMS mean?
HORNBEAMS noun- A tree of the genus Carpinus, having a smooth gray bark and a ridged trunk, the wood being white and very hard, common along the banks of streams in the United States.
- A hop hornbeam (Ostrya virginiana).
- The wood of these trees.