The most common and most likely answer to this crossword clue is the 8 letter word RIGIDITY
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Buddy explains!
The straight part
We know that "stiffness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stiffness = rigidity.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 RIGIDITY mean?
RIGIDITY noun- The quality or state of being rigid; want of pliability; the quality of resisting change of form; the amount of resistance with which a body opposes change of form.
- Stiffness of appearance or manner; want of ease or elegance.
- Stickiness (of prices/wages etc.). Describing the tendency of prices and money wages to adjust to changes in the economy with a certain delay.