A very loud broadcast is happening! (6)
I believe the answer is:
affair
'is happening' is the definition.
(I've seen this before)
'a very loud broadcast' is the wordplay.
'very loud' becomes 'ff' (musical abbreviation).
'broadcast' becomes 'air' (synonyms).
'a'+'ff'+'air'='AFFAIR'
(Other definitions for affair that I've seen before include "Secret liaison" , "Concern (maybe extra-marital?)" , "Matter or concern, perhaps of the heart" , "A matter, perhaps of love" , "Liaison or intrigue" .)