Vaccine fluid (5)
I believe the answer is:
serum
'vaccine fluid' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for serum that I've seen before include "Muser in blood" , "Watery fluid from animal tissue" , "Constituent of blood" , "Liquid separating from coagulating blood" , "Watery part of blood" .)