The music they peddled, captivated industry chief, which is very hard (4,5)
I believe the answer is:
rock solid
'the music they peddled captivated industry chief which' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'very hard' is the wordplay.
'very' becomes 'rock' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'hard' becomes 'solid' (similar in meaning).
'rock'+'solid'='ROCK-SOLID'
'is' is the link.
Can you help me to learn more?
(Other definitions for rock solid that I've seen before include "Completely firm" , "Absolutely settled and reliable" , "Guaranteed" .)