New song about Disc World (7)
I believe the answer is:
neptune
'world' is the definition.
(I've seen this before)
'new song about disc' is the wordplay.
'new' becomes 'n'.
'song' becomes 'tune' (I've seen this before**).
'about' is a charade indicator (letters next to each other).
'disc' becomes 'ep'.
'tune' after 'ep' is 'eptune'.
'n'+'eptune'='NEPTUNE'
(Other definitions for neptune that I've seen before include "Remote planet" , "one of the Planets" , "Third largest of the planets, and sea god" , "One distanced from The Sun" , "Roman sea-god" .)