Type of music in decline (5)
I believe the answer is:
indie
'type of music' is the definition.
(I've seen this before)
'in decline' is the wordplay.
'decline' becomes 'die' (I've seen this in other clues).
'in'+'die'='INDIE'
(Other definitions for indie that I've seen before include "Autonomous film co." , "Not a mainstream group" , "Pop genre" , "minor label" , "(Pop group) not affiliated with a major record company" .)