The most common and most likely answer to this crossword clue is the 6 letter word EVENER
I've seen this crossword clue in Financial Times
E
V
E
N
E
R
More clues leading to the result EVENER
Buddy explains!

The straight part
We know that "Flatter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flatter = evener.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 EVENER mean?
EVENER noun- One who, or that which, makes even.
- In vehicles, a swinging crossbar, to the ends of which other crossbars, or whiffletrees, are hung, to equalize the draught when two or three horses are used abreast.
- A raddle (instrument used by weavers).