Target of an annual vaccine (3)
I believe the answer is:
flu
'target of an annual vaccine' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for flu that I've seen before include "Common ailment" , "Common annual illness" , "Viral infection (colloq.)" , "Winter disease, awfully avian" , "Viral illness" .)