Illness went quickly, we hear (3)
I believe the answer is:
flu
'illness' is the definition.
(flu is a kind of illness)
'went quickly we hear' is the wordplay.
'went quickly' becomes 'flew' (the time flew by).
'we hear' shows a homophone (sound like).
'flew' is a homophone of 'FLU'.
(Other definitions for flu that I've seen before include "Common annual illness" , "Viral illness" , "Viral infection (abbr)" , "Viral infection (colloq.)" , "Common infection" .)