Coagulate, as blood (4)
I believe the answer is:
clot
'coagulate as blood' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for clot that I've seen before include "(Of blood) coagulate" , "Stupid person - lump of blood" , "Clumsy person" , "Fool - congeal" , "Semi-solid mass -- dunce" .)