The most common and most likely answer to this crossword clue is the 8 letter word TRAMPLED
I've seen this crossword clue in Financial Times
T
R
A
M
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D
More clues leading to the result TRAMPLED
Buddy explains!

The straight part
We know that "Flattened" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flattened = trampled.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 TRAMPLED mean?
TRAMPLED verb- To crush something by walking on it.
- (by extension) To treat someone harshly.
- To walk heavily and destructively.
- (by extension) To cause emotional injury as if by trampling.