What's not mainstream -- popular before getting to fade away (5)
I believe the answer is:
indie
'what's not mainstream' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'popular before getting to fade away' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'before getting' means one lot of letters go next to another.
'to fade away' becomes 'die' (I've seen this in another clue).
'in'+'die'='INDIE'
Can you help me to learn more?
(Other definitions for indie that I've seen before include "Unaffiliated (pop record label)" , "Independent record company" , "Non-mainstream film company" , "(Pop group) not affiliated with a major record company" , "Branch of pop music" .)