Common viral infection (3)
'common viral infection' 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 annual illness" , "Viral infection (abbr)" , "Common infection" , "Mild infection" , "Common ailment" .)
(Other definitions for influenza that I've seen before include "Respiratory complaint" , "Nasty viral disease, usually of winter" , "Highly infectious viral disease" , "Illness once called grippe" , "Viral infection" .)