The most common and most likely answer to this crossword clue is the 14 letter word DADDY-LONGLEGS
D
A
D
D
Y
-
L
O
N
G
L
E
G
S
More clues leading to the result DADDY-LONGLEGS
Buddy explains!
The straight part
We know that "Fly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fly = daddy-longlegs.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.