Clotting agent (8)
I believe the answer is:
platelet
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">clotting agent</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)