The most common and most likely answer to this crossword clue is the 6 letter word *PANDA
I've seen this crossword clue in The Guardian
*
P
A
N
D
A
Buddy explains!
The straight part
We know that "an endangered species/an endangered species" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an endangered species/an endangered species = *panda.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 *PANDA mean?
*PANDA noun- The red panda (Ailurus fulgens), a small raccoon-like animal of northeast Asia with reddish fur and a long, ringed tail.
- Short for giant panda.
- (law enforcement) Short for panda car.
- A brahmin who acts as the hereditary superintendent of a particular ghat or temple, and is regarded as knowledgeable in matters of genealogy and ritual.