The most common and most likely answer to this crossword clue is the 10 letter word FLAGELLATE
I've seen this crossword clue in The Times
F
L
A
G
E
L
L
A
T
E
More clues leading to the result FLAGELLATE
Buddy explains!
The straight part
We know that "Thrash" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case thrash = flagellate.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 FLAGELLATE mean?
FLAGELLATE noun- Any organism that has flagella.
- To whip or scourge.
- Resembling a whip.
- Having flagella.