Minute particle also called a thrombocyte crucial to the clotting of the blood of vertebrates (8)
I believe the answer is:
platelet
(Other definitions for platelet that I've seen before include "Particle of protoplasm, essential for blood clotting - tell a pet (anag)" , "Small disc in the blood" , "Disc-shaped blood cell" , "Cell fragment in the blood, important for clotting" , "something in the arteries" .)