The most common and most likely answer to this crossword clue is the 8 letter word BUMPIEST
I've seen this crossword clue in The Sunday Telegraph
B
U
M
P
I
E
S
T
Buddy explains!
The straight part
We know that "that is most rough" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that is most rough = bumpiest.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 BUMPIEST mean?
BUMPIEST adjective- Rough; jumpy; causing or characterized by jolts and irregular movements
- Covered with or full of bumps