Viral infection was airborne, according to radio report (3)
I believe the answer is:
flu
'viral infection' is the definition.
(I've seen this before)
'was airborne according to radio report' is the wordplay.
'was airborne' becomes 'flew' (flew in the air).
'according to radio report' shows a homophone (sound like).
'flew' sounds like 'FLU'.
(Other definitions for flu that I've seen before include "A bad winter bug" , "Viral infection (abbr)" , "Swine --; gastric --" , "Viral illness" , "Common ailment" .)