It's in your blood (5)
I believe the answer is:
serum
'it's in your blood' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for serum that I've seen before include "Watery part of blood" , "Amber-coloured fluid in the blood" , "Watery fluid from animal tissue" , "Part of blood; vaccine" , "Liquid separating from coagulating blood" .)