The most common and most likely answer to this crossword clue is the 6 letter word GANDER
I've seen this crossword clue in The Guardian
G
A
N
D
E
R
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 = gander.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 GANDER mean?
GANDER noun- A male goose.
- A fool, simpleton.
- (used only with “have”, “get” and “take”) A glance, look.
- A man living apart from his wife.
- Ramble, wander