Mass of coagulated blood (4)
I believe the answer is:
clot
'mass of coagulated blood' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for clot that I've seen before include "Mass of coagulated blood" , "Twit; blood problem" , "Fool; solidified blood" , "Thrombus - fool" , "Coagulated blood; twit" .)