The most common and most likely answer to this crossword clue is the 5 plus 9 letter word HORSE CHESTNUTS
I've seen this crossword clue in The Daily Telegraph
H
O
R
S
E
C
H
E
S
T
N
U
T
S
More clues leading to the result HORSE CHESTNUTS
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 = horse chestnuts.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.