The most common and most likely answer to this crossword clue is the 10 letter word BLUEBOTTLE
I've seen this crossword clue in Independent
B
L
U
E
B
O
T
T
L
E
More clues leading to the result BLUEBOTTLE
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 = bluebottle.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 BLUEBOTTLE mean?
BLUEBOTTLE noun- Any of various blowflies of the genus Calliphora that have an iridescent metallic-blue body and make a loud buzzing noise when flying.
- A marine jellyfish of the genus Physalia, which includes Physalia physalis, the Portuguese man-of-war, and Physalia utriculus, the Pacific man-of-war; a man-of-war.
- A cornflower, a plant that grows in grain fields, Centaurea cyanus, with blue flowers resembling bottles.
- A blue ant, Diamma bicolor, a parasitic wasp native to Australia.
- Any of various large papilionid butterflies of the genus Graphium. Also called triangles.
- A police officer.