The most common and most likely answer to this crossword clue is the 6 letter word REWILD
I've seen this crossword clue in The Guardian
R
E
W
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Buddy explains!
The straight part
We know that "return to natural habitat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case return to natural habitat = rewild.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 REWILD mean?
REWILD verb- (conservation) To return an area to a more wild state, especially to repopulate it with wild animals.