Blood fluid (5)
I believe the answer is:
'blood fluid' 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" , "Blood component containing antibodies, used in immunisation" , "Antitoxin derived from blood" , "Liquid separating from coagulating blood" , "Watery liquid as from blister, or fluid in blood" .)