Crossword Genius

Where papers are kept in detective's investigation about plot (8,4)

Ross

I believe the answer is:

dispatch case

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

'detective's investigation about plot' is the wordplay.
'detective's' becomes 'di's' (Detective Inspector).
'investigation' becomes 'case'.
'about' is a charade indicator (letters next to each other).
'plot' becomes 'patch' (synonyms).
'case' put after 'patch' is 'patchcase'.
'dis'+'patchcase'='DISPATCH CASE'

'in' acts as a link.

Can you help me to learn more?

(Another definition for dispatch case that I've seen is " documents found here".)

I've seen this clue in The Times.
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