The most common and most likely answer to this crossword clue is the 9 letter word CYPRESSES
I've seen this crossword clue in The Daily Telegraph
C
Y
P
R
E
S
S
E
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 = cypresses.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 CYPRESSES mean?
CYPRESSES noun- An evergreen coniferous tree with flattened shoots bearing small scale-like leaves, whose dark foliage is sometimes associated with mourning, in family Cupressaceae, especially the genera Cupressus and Chamaecyparis.