Clotting agent (8)
I believe the answer is:
platelet
'clotting agent' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for platelet that I've seen before include "Small disc in the blood" , "Disc-shaped blood cell" , "Particle of protoplasm, essential for blood clotting - tell a pet (anag)" , "Disc found in blood" , "Item in blood involved in clotting" .)