The most common and most likely answer to this crossword clue is the 6 letter word WAVIER
I've seen this crossword clue in The Times
W
A
V
I
E
R
Buddy explains!

The straight part
We know that "comparatively less flat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case comparatively less flat = wavier.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 WAVIER mean?
WAVIER adjective- Rising or swelling in waves.
- Full of waves.
- Moving to and fro; undulating.
- Having wave-like shapes on its border or surface; waved.
- (of a margin) Moving up and down relative to the surface; undulate.
- Undé, in a wavy line; applied to ordinaries, or division lines.
- Drunk