The most common and most likely answer to this crossword clue is the 5 letter word LLANO
I've seen this crossword clue in The Times
L
L
A
N
O
More clues leading to the result LLANO
- Personal life somewhat on the up, it’s plain
- Look around lake and an extensive 24
- Hidden by hill, another plain
- Backward about everything, it’s plain
- Some fill a novel — that’s plain
- Looking back, acceptable and entirely plain
- Cricket side everyone backs, it's plain
- Plain to see on reflection where general obligation falls?
- I refuse to go on backing everyone: that’s flat!
- “Plain” Cinderella nobleman embraces
Buddy explains!

The straight part
We know that "that’s flat!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that’s flat! = llano.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 LLANO mean?
LLANO noun- A plain or steppe in parts of Latin America.