Antitoxin (5)
I believe the answer is:
serum
'antitoxin' 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 "Antitoxin derived from blood" , "Blood component containing antibodies, used in immunisation" , "Part of blood; vaccine" , "Watery part of blood" , "Muser in blood" .)