The most common and most likely answer to this crossword clue is the 7 letter word STEEPER
S
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P
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Buddy explains!

The straight part
We know that "harder to climb" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case harder to climb = steeper.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 STEEPER mean?
STEEPER adjective- Of a near-vertical gradient; of a slope, surface, curve, etc. that proceeds upward at an angle near vertical.
- Expensive
- Difficult to access; not easy reached; lofty; elevated; high.
- (of the rake of a ship's mast, or a car's windshield) resulting in a mast or windshield angle that strongly diverges from the perpendicular
- A vessel, vat, or cistern in which things are steeped.