What’s more or less popular music, full of energy (7)
I believe the answer is:
besides
'what's more' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for besides that I've seen before include "Apart from" , "Moreover" , "In addition, as well" , "See bids (anag)" , "also is available" .)