The most common and most likely answer to this crossword clue is the 11 letter word HUMMINGBIRD
I've seen this crossword clue in The Daily Telegraph
H
U
M
M
I
N
G
B
I
R
D
More clues leading to the result HUMMINGBIRD
Buddy explains!
The straight part
We know that "flier?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flier? = hummingbird.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 HUMMINGBIRD mean?
HUMMINGBIRD noun- Any of various small American birds in the family Trochilidae that have the ability to hover.