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Letter bound to have shown sympathy (6)

Ross
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'letter bound' is the definition.
'penned' can be an answer for 'bound' (thesaurus). I am unsure of the 'letter' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for penned that I've seen before include "Wrote; confined" , "Confined; written" , "Shut in" , "in Reading Gaol, perhaps" , "in captivity" .)

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