Final, formal brushoff for Augustus or Gwen? (4,4,6)

I believe the answer is:

dear john letter

But I can't explain it at all!

(Other definitions for dear john letter that I've seen before include "Rejection" , "chucking post" , "note ending a relationship" , "Final, formal brush-off" , "for Augustus or Gwen?" .)

I've seen this clue in The Times.

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