Artery blocker (4)
I believe the answer is:
clot
'artery blocker' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for clot that I've seen before include "Thick lump - thick person" , "Semi-solid mass -- dunce" , "Coagulate -- 1 across person" , "3 [DUMMY]" , "(Of blood) coagulate" .)