The most common and most likely answer to this crossword clue is the 6 letter word SLOTHS
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More clues leading to the result SLOTHS
Buddy explains!

The straight part
We know that "Tree-dwellers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tree-dwellers = sloths.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 SLOTHS mean?
SLOTHS noun- Laziness; slowness in the mindset; disinclination to action or labour.
- A herbivorous, arboreal South American mammal of the families Megalonychidae and Bradypodidae, noted for its slowness and inactivity.
- A collective term for a group of bears.