Copper is on the walkie-talkie for so long (3,3)

I believe the answer is:

see you

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'for so long' is the definition.
(I've seen this in another clue)

I can't explain the remainder of the clue.

(Other definitions for see you that I've seen before include "poker-playing expression?" , "Cheers" , "Bye!" , "Later we'll meet" , "Casual and informal goodbye" .)

I've seen this clue in The Times and The Sunday Times.

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