Everyone into rotten tune (6)
I believe the answer is:
ballad
'tune' is the definition.
(type of song)
'everyone into rotten' is the wordplay.
'everyone' becomes 'all' (all people).
'into' is an insertion indicator.
'rotten' becomes 'bad' (similar in meaning).
'all' inserted inside 'bad' is 'BALLAD'.
(Other definitions for ballad that I've seen before include "Slow romantic song" , "Poem telling a story" , "romance" , "A simple or old song" , "Folk song" .)