The most common and most likely answer to this crossword clue is the 9 letter word CONGEALED
I've seen this crossword clue in The Daily Telegraph
C
O
N
G
E
A
L
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 = congealed.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 CONGEALED mean?
CONGEALED verb- To change from a liquid to solid state perhaps by cold
- To coagulate, make curdled or semi-solid as gel or jelly
- To make rigid or immobile
- To become congealed, solidify
- Frozen.
- Viscid, coagulated; jelly-like, unusually thick (of a liquid).