The most common and most likely answer to this crossword clue is the 10 letter word PROSTRATED
I've seen this crossword clue in The Guardian
P
R
O
S
T
R
A
T
E
D
Buddy explains!

The straight part
We know that "flat out" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flat out = prostrated.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 PROSTRATED mean?
PROSTRATED verb- To lie flat or face-down.
- To throw oneself down in submission.
- To cause to lie down, to flatten.
- To overcome or overpower.