Pass by popular music genre (5)
I believe the answer is:
indie
'music genre' is the definition.
'pass by popular' is the wordplay.
'pass' becomes 'die' (as in passing away).
'by' means one lot of letters go next to another.
'popular' becomes 'in' ('in' can mean trendy).
'die' put after 'in' is 'INDIE'.
(Other definitions for indie that I've seen before include "(Pop group) not affiliated with a major record company" , "Non-mainstream record label" , "Category of pop music" , "(Pop group) not affiliated to a major recording company" , "Not signed up to a major record company" .)