The most common and most likely answer to this crossword clue is the 7 letter word SUMACHS
I've seen this crossword clue in The Guardian
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More clues leading to the result SUMACHS
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The straight part
We know that "trees/trees" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case trees/trees = sumachs.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.
Crossword Answer definitions
SUMACHS noun- Any of various shrubs or small trees of the genus Rhus and other genera in Anacardiaceae, particularly the elm-leaved sumac, Sicilian sumac, or tanner's sumac (Rhus coriaria).
- Dried and chopped-up leaves and stems of a plant of the genus Rhus, particularly the tanner's sumac (see sense 1), used for dyeing and tanning leather or for medicinal purposes.
- A sour spice popular in the Eastern Mediterranean, made from the berries of tanner's sumac.