The most common and most likely answer to this crossword clue is the 8 letter word HARDNESS
I've seen this crossword clue in The Guardian
H
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Buddy explains!

The straight part
We know that "Difficulty" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case difficulty = hardness.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 HARDNESS mean?
HARDNESS noun- The quality of being hard.
- An instance of this quality; hardship.
- The quantity of calcium carbonate dissolved in water, usually expressed in parts per million (ppm).
- The resistance to scratching, cutting, indentation or abrasion of a metal or other solid material.
- The penetrating ability of electromagnetic radiation, such as x-rays; generally, the shorter the wavelength, the harder and more penetrating the radiation.
- The measure of resistance to damage of a facility, equipment, installation, or telecommunications infrastructure when subjected to attack.