Everything contained in terrible song (6)
I believe the answer is:
ballad
'song' is the definition.
(ballad is a kind of song)
'everything contained in terrible' is the wordplay.
'everything' becomes 'all' (I've seen this before).
'contained in' indicates putting letters inside.
'terrible' becomes 'bad' (similar in meaning).
'all' placed inside 'bad' is 'BALLAD'.
(Other definitions for ballad that I've seen before include "Simple song or poem" , "romance" , "Narrative poem or song" , "Song type" , "Poem telling a story" .)