With which to mime rock song about gratuitous dropouts (3,6)
I believe the answer is:
air guitar
'with which to mime rock' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'song about gratuitous dropouts' is the wordplay.
'song' becomes 'air' (air can mean a piece of music).
'about gratuitous dropouts' becomes 'guitar' (I can't justify this - if you can you should believe this answer much more).
'air'+'guitar'='AIR GUITAR'
Can you help me to learn more?
(Other definitions for air guitar that I've seen before include "Imaginary instrument played by rock fans!" , "Imaginary instrument that one pretends to play" , "Imaginary stringed instrument" , "Instrumental object that's non-existent" , "Just imagine playing this" .)