The most common and most likely answer to this crossword clue is the 11 letter word COAGULATION
C
O
A
G
U
L
A
T
I
O
N
Buddy explains!

The straight part
We know that "clot" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case clot = coagulation.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.
Crossword Answer definitions
COAGULATION noun- The precipitation of suspended particles as they increase in size (by any of several physical or chemical processes)
- The process by which blood forms solid clots.
- Similar solidification of other materials (e.g. of tofu).