Doctor to unwind, absorbing hospital casebook? (9)

I believe the answer is:

whodunnit

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'casebook?' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'doctor to unwind absorbing hospital' is the wordplay.
'doctor' is an anagram indicator.
'absorbing' indicates putting letters inside.
'hospital' becomes 'H' (abbreviation for hospital on maps).
'to'+'unwind'='tounwind'
'tounwind' anagrammed gives 'wodunnit'.
'wodunnit' placed around 'h' is 'WHODUNNIT'.

Can you help me to learn more?

(Other definitions for whodunnit that I've seen before include "literary mystery" , "book" , "crime novel" , "yarn unravelled finally?" , "Mystery drama" .)

I've seen this clue in The Times.

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