Copper on radio for so long (3,3)
I believe the answer is:
see you
'for so long' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for see you that I've seen before include "Bye!" , "Cheers" , "Informal kind of au revoir - with visual intent" , "Casual and informal goodbye" , "Later we'll meet" .)