The most common and most likely answer to this crossword clue is the 7 letter word SWATTED
I've seen this crossword clue in The Daily Telegraph
S
W
A
T
T
E
D
Buddy explains!
The straight part
We know that "battered flies" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case battered flies = swatted.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 SWATTED mean?
SWATTED verb- To beat off, as insects; to bat, strike, or hit.
- To illegitimately provoke a SWAT assault upon (someone).