The most common and most likely answer to this crossword clue is the 10 letter word SWITCHBACK
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Buddy explains!

The straight part
We know that "very hilly road" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case very hilly road = switchback.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 SWITCHBACK mean?
SWITCHBACK noun- A zigzag path, road or railway track; especially a railway track in which the train travels in a reverse direction at each switch
- A hairpin bend.
- A roller coaster.
- A route with alternate sharp ascents and descents.
- (of a path etc) To zigzag.