Curdle (4)
I believe the answer is:
clot
'curdle' is the definition.
(I know that curdle can be written as clot)
This is all the clue.
(Other definitions for clot that I've seen before include "A lump in a blood vessel" , "Coagulate -- 1 across person" , "Semi-solid mass -- dunce" , "Mass of material" , "3 [DUMMY]" .)