Flu fighter
(Other definitions for shot that I've seen before include "Gunned down - go - guess" , "Try -- ammunition" , "Slug" , "Knackered" , "Crack" .)
'flu fighter' 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 "Inoculation liquid" , "Antidote" , "Watery liquid as from blister, or fluid in blood" , "Constituent of blood" , "Blood component containing antibodies, used in immunisation" .)