The most common and most likely answer to this crossword clue is the 10 letter word COAGULATED
I've seen this crossword clue in The Sunday Telegraph
C
O
A
G
U
L
A
T
E
D
Buddy explains!

The straight part
We know that "set" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case set = coagulated.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 COAGULATED mean?
COAGULATED verb- To become congealed; to convert from a liquid to a semisolid mass.
- To cause to congeal.
- Subject to coagulation.