Run down to follow trendy pop genre (5)
I believe the answer is:
indie
'pop genre' is the definition.
(I've seen this before)
'run down to follow trendy' is the wordplay.
'run down' becomes 'die' (run down can mean to decline or die).
'to follow' means one lot of letters go next to another (some letters go after or follow others).
'trendy' becomes 'in' ('in' can mean fashionable).
'die' put after 'in' is 'INDIE'.
(Other definitions for indie that I've seen before include "Category of pop music" , "Pop genre" , "Branch of pop music" , "Specialist record company" , "minor label" .)