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Letter bound to have shown sympathy (6)

Ross
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'letter' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

I don't understand the remainder of the clue.

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(Other definitions for buster that I've seen before include "-- Keaton, US silent film actor" , "Keaton or Crabbe" , "-- Keaton, comic actor" , "- - Keaton, silent star" , "Form of address to man" .)

I've seen this clue in The Globe and Mail.
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