The most common and most likely answer to this crossword clue is the 12 letter word GENEALOGISTS
I've seen this crossword clue in The Daily Telegraph
G
E
N
E
A
L
O
G
I
S
T
S
Buddy explains!
The straight part
We know that "Tree experts" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tree experts = genealogists.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 GENEALOGISTS mean?
GENEALOGISTS noun- A person who studies or practises genealogy, an expert in genealogy.