Edge sounds down but it's not a real performance (3-5)
I believe the answer is:
lip-synch
'it's not a real performance' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'edge sounds down' is the wordplay.
'edge' becomes 'lip' (lip is a kind of edge).
'sounds down' becomes 'synch' (I am not sure about this - if you are sure you should believe this answer much more).
'lip'+'synch'='LIP-SYNCH'
'but' is the link.
Can you help me to learn more?
(Other definitions for lip-synch that I've seen before include "Mouth pre-recorded words, eg on film" , "Pretend to be singing by mouthing the words" , "Speak to a pre-recorded soundtrack" .)