Quietly boast about good correspondence (7)

I believe the answer is:

postbag

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'correspondence' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'quietly boast about good' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'about' indicates anagramming the letters.
'good' becomes 'g' (abbreviation).
'boast' anagrammed gives 'ostba'.
'p'+'ostba'+'g'='POSTBAG'

Can you help me to learn more?

(Other definitions for postbag that I've seen before include "carrier for Royal Mail" , "All delivered" , "What one receives collectively" , "Letters received" , "one's letters" .)

I've seen this clue in The Daily Express.

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