The most common and most likely answer to this crossword clue is the 8 letter word STARCHES
I've seen this crossword clue in The Daily Telegraph
S
T
A
R
C
H
E
S
Buddy explains!
The straight part
We know that "Stiffens" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stiffens = starches.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 STARCHES mean?
STARCHES noun- A widely diffused vegetable substance found especially in seeds, bulbs, and tubers, and extracted (as from potatoes, corn, rice, etc.) as a white, glistening, granular or powdery substance, without taste or smell, and giving a very peculiar creaking sound when rubbed between the fingers. It is used as a food, in the production of commercial grape sugar, for stiffening linen in laundries, in making paste, etc.
- (nutrition) Carbohydrates, as with grain and potato based foods.
- A stiff, formal manner; formality.
- Any of various starch-like substances used as a laundry stiffener
- To apply or treat with laundry starch, to create a hard, smooth surface.