Song about everything being rotten (6)
I believe the answer is:
ballad
'song' is the definition.
(I know that ballad is a type of song)
'about everything being rotten' is the wordplay.
'about' means one lot of letters goes inside another.
'everything' becomes 'all' (all things).
'being rotten' becomes 'bad' ('bad' can be similar in meaning to 'rotten').
'all' placed into 'bad' is 'BALLAD'.
(Other definitions for ballad that I've seen before include "Narrative song of popular origin" , "Simple song or poem" , "Slow sentimental song" , "Love song" , "Slow romantic song" .)