Antidote (5)
I believe the answer is:
serum
'antidote' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for serum that I've seen before include "Antidote" , "Antitoxin derived from blood" , "Watery liquid as from blister, or fluid in blood" , "Inoculation liquid" , "Blood component containing antibodies, used in immunisation" .)