The most common and most likely answer to this crossword clue is the 3 letter word ASH
A
S
H
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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 = ash.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 ASH mean?
ASH noun- The solid remains of a fire.
- The nonaqueous remains of a material subjected to any complete oxidation process.
- Fine particles from a volcano, volcanic ash.
- (in the plural) Human (or animal) remains after cremation.
- What remains after a catastrophe.
- A gray colour, like that of ash.
- To reduce to a residue of ash. See ashing.
- To hit the end off of a burning cigar or cigarette.
- To hit the end off (a burning cigar or cigarette).
- (mostly used in the passive) To cover newly-sown fields of crops with ashes.
- A shade tree of the genus Fraxinus.
- The wood of this tree.
- The traditional name for the ae ligature (æ), as used in Old English.