Playing the piano backstage (2,3,5)
I believe the answer is:
on the quiet
'backstage' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
'playing the piano' is the wordplay.
'playing' becomes 'on' (on can mean playing or performing).
'piano' becomes 'quiet' (I can't explain this - if you can you should believe this answer much more).
'on'+'the'+'quiet'='ON THE QUIET'
Can you help me to learn more?
(Other definitions for on the quiet that I've seen before include "Unobtrusively" , "Surreptitiously" , "Secretly" .)