The most common and most likely answer to this crossword clue is the 8 letter word THRESHED
I've seen this crossword clue in The Times
T
H
R
E
S
H
E
D
Buddy explains!

The straight part
We know that "flailed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flailed = threshed.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 THRESHED mean?
THRESHED verb- To separate the grain from the straw or husks (chaff) by mechanical beating, with a flail or machinery.
- To beat soundly, usually with some tool such as a stick or whip; to drub.