The most common and most likely answer to this crossword clue is the 5 letter word PINON
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More clues leading to the result PINON
Buddy explains!

The straight part
We know that "tree" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tree = pinon.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 PINON mean?
PINON noun- Any of several species of North American pines in Pinus subsect. Cembroides that bear edible seeds (pine nuts), especially Pinus edulis; the nut pine.
- A pine nut.