The most common and most likely answer to this crossword clue is the 6 letter word DEODAR
I've seen this crossword clue in Independent
D
E
O
D
A
R
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 = deodar.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 DEODAR mean?
DEODAR noun- Cedrus deodara, a type of cedar tree native to the western Himalayas.