Antitoxin (5)
I believe the answer is:
serum
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">antitoxin</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for serum that I've seen before include "Antitoxin derived from blood" , "Blood component containing antibodies, used in immunisation" , "Part of blood; vaccine" , "Watery part of blood" , "Muser in blood" .)