The most common and most likely answer to this crossword clue is the 5 letter word RAZED
I've seen this crossword clue in Financial Times
R
A
Z
E
D
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 = razed.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 RAZED mean?
RAZED verb- To demolish; to level to the ground.
- To scrape as if with a razor.
- Slashed or striped in patterns.