The most common and most likely answer to this crossword clue is the 4 letter word YEWS
Y
E
W
S
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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 = yews.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 YEWS mean?
YEWS noun- A species of coniferous tree, Taxus baccata, with dark-green flat needle-like leaves and seeds bearing red arils, native to western, central and southern Europe, northwest Africa, northern Iran and southwest Asia.
- (by extension) Any tree or shrub of the genus Taxus.
- Other conifers resembling plants in genus Taxus
- The wood of the such trees.
- A bow for archery, made of yew wood.