The most common and most likely answer to this crossword clue is the 6 letter word PIGNUT
I've seen this crossword clue in The Guardian
P
I
G
N
U
T
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 = pignut.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 PIGNUT mean?
PIGNUT noun- The edible tuber of Conopodium majus, native to western Europe.
- Any of various types of hickory or their fruits; a hognut.
- Simmondsia chinensis, jojoba.