The most common and most likely answer to this crossword clue is the 4 letter word HYLA
H
Y
L
A
Buddy explains!
The straight part
We know that "Tree frog" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tree frog = hyla.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 HYLA mean?
HYLA noun- A tree frog of the genus Hyla.
- Matter
- The first matter of the cosmos, from which the four elements arose, according to the doctrines of Empedocles and Aristotle.