Non-mainstream labels pass to enter popular stage at the front (6)
I believe the answer is:
indies
'non-mainstream labels' is the definition.
I can't judge whether this defines the answer.
'pass to enter popular stage at the front' is the wordplay.
'pass' becomes 'die' (as in passing away).
'to enter' indicates putting letters inside.
'popular' becomes 'in' ('in' can mean trendy).
'at the front' indicates taking the first letters.
The first letter of 'stage' is 's'.
'in'+'s'='ins'
'die' placed into 'ins' is 'INDIES'.
Can you help me to learn more?
(Other definitions for indies that I've seen before include "small film companies" , "SE Asia from India eastwards" , "They can be East or West" .)