The most common and most likely answer to this crossword clue is the 9 letter word GREENWOOD
I've seen this crossword clue in The Times
G
R
E
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N
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Buddy explains!
The straight part
We know that "lot of trees in leaf" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lot of trees in leaf = greenwood.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 GREENWOOD mean?
GREENWOOD noun- A forest in full leaf, as in summer.
- Wood that is green; in other words, not seasoned.
- Certain half-shrubby species of genista.