Whey (5)
I believe the answer is:
serum
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">whey</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 serum that I've seen before include "Antidote" , "Inoculation liquid" , "Part of blood; vaccine" , "Constituent of blood" , "Muser in blood" .)