The most common and most likely answer to this crossword clue is the 8 letter word STIFFENS
I've seen this crossword clue in The Times
S
T
I
F
F
E
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S
More clues leading to the result STIFFENS
Buddy explains!

The straight part
We know that "gets harder" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gets harder = stiffens.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 STIFFENS mean?
STIFFENS verb- To make stiff.
- To become stiff.