Everyone will participate in rotten song (6)
I believe the answer is:
ballad
'song' is the definition.
(ballad is a kind of song)
'everyone will participate in rotten' is the wordplay.
'everyone' becomes 'all' (all people).
'will participate in' means one lot of letters goes inside another.
'rotten' becomes 'bad' (similar in meaning).
'all' placed into 'bad' is 'BALLAD'.
(Other definitions for ballad that I've seen before include "Simple song or poem" , "Form of verse" , "Narrative song of popular origin" , "delivered by singer" , "romance" .)