The most common and most likely answer to this crossword clue is the 10 letter word JACARANDAS
I've seen this crossword clue in The Guardian
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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 = jacarandas.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
JACARANDAS noun- Any of several trees, of the genus Jacaranda, native to tropical South America, that have pale purple, funnel-shaped flowers.
- The hard, dark wood of these trees.
- A trade name for similar hardwood timber from certain species of Dalbergia, notably Dalbergia frutescens, Dalbergia nigra and Dalbergia refusa.