Blood clot
I believe the answer is:
thrombosis
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">blood clot</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for thrombosis that I've seen before include "Blood clot" , "Complaint" .)