The most common and most likely answer to this crossword clue is the 9 plus 6 letter word PASSENGER PIGEON
I've seen this crossword clue in Independent
P
A
S
S
E
N
G
E
R
P
I
G
E
O
N
More clues leading to the result PASSENGER PIGEON
- Former migrant who doesn't pull his weight, a brute European working
- High-flyer once seen going with papers for shredding
- Pope’s gangrene is treated by former high-flyer
- Craft seen with ageing props — it’s no longer flying
- Bird, as revealed by sad page in press, gone
- E bird – Grand Peregrine not right – disoriented in relay
- Traveller taking responsibility for vanished bird
- Poe seen grasping bizarre, extinct bird
Buddy explains!
The straight part
We know that "High-flyer once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case high-flyer once = passenger pigeon.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.