Addition to music player disco regularly used (5)
I believe the answer is:
lyric
'addition to music' is the definition.
(lyrics are sung over music)
'player disco regularly used' is the wordplay.
'regularly used' indicates alternate letters (letters selected on a regular basis).
'player'+'disco'='playerdisco'
The alternate letters of 'playerdisco' are 'LYRIC'.
(Other definitions for lyric that I've seen before include "One barred from native country" , "Expressing the poet's emotions" , "Song's text" , "Of poetry that expresses thoughts and feelings" , "A song or poem" .)