The most common and most likely answer to this crossword clue is the 7 letter word TABULAR
I've seen this crossword clue in The Times
T
A
B
U
L
A
R
More clues leading to the result TABULAR
Buddy explains!

The straight part
We know that "flat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flat = tabular.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 TABULAR mean?
TABULAR adjective- Having a flat, plane surface
- Organized as a table or list
- Calculated by means of a table
- Tending to split into thin flat pieces, such as slate