The most common and most likely answer to this crossword clue is the 9 letter word MOSQUITOS
I've seen this crossword clue in The Daily Telegraph
M
O
S
Q
U
I
T
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S
Buddy explains!
The straight part
We know that "flies" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flies = mosquitos.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 MOSQUITOS mean?
MOSQUITOS noun- A small flying insect of the family Culicidae, the females of which bite humans and animals and suck blood, leaving an itching bump on the skin, and sometimes carrying diseases like malaria and yellow fever.
- To fly close to the ground, seemingly without a course.