The most common and most likely answer to this crossword clue is the 7 letter word BEECHES
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More clues leading to the result BEECHES
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 = beeches.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 BEECHES mean?
BEECHES noun- A tree of the genus Fagus having a smooth, light grey trunk, oval, pointed leaves and many branches.
- The wood of the beech tree.