Vital fluid (5)
I believe the answer is:
blood
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">vital fluid</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 blood that I've seen before include "It contains plasma and platelets" , "vital liquid" , "Vein filler" , "Red fluid" , "It's said to be thicker than water" .)