Seasonal ailment (3)
I believe the answer is:
flu
'seasonal ailment' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for flu that I've seen before include "Common ailment" , "Winter disease, awfully avian" , "Highly contagious viral disease" , "Viral infection (colloq.)" , "Minor ailment" .)