Cut in if they're still rocking! (3,3)
I believe the answer is:
the who
'they're still rocking' is the definition.
'cut in if' is the wordplay.
'cut' becomes 'hew' (I've seen this before).
'in' indicates putting letters inside.
'if' becomes 'tho' (I can't justify this - if you can you should give a lot more credence to this answer).
'hew' inserted into 'tho' is 'THE WHO'.
(Other definitions for the who that I've seen before include "Pop group (Tommy, Quadrophenia)" , "Tommy, Quadrophenia rock group" , "Loud rock band" , "British pop group" , "60s rock group" .)