Quietly go bats, organising 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 see how one could define the other.

'quietly go bats organising' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'organising' indicates an anagram.
'go'+'bats'='gobats'
'gobats' anagrammed gives 'ostbag'.
'p'+'ostbag'='POSTBAG'

Can you help me to learn more?

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

I've seen this clue in The Daily Express.

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