Sound diplomacy after case of incarceration (6)
I believe the answer is:
intact
'sound' is the definition.
(I've seen this before)
'diplomacy after case of incarceration' is the wordplay.
'diplomacy' becomes 'tact' (I've seen this before).
'after' says to put letters next to each other.
'case of' says to hollow out the word (remove centre letters) (outsides of).
'incarceration' with its middle taken out is 'in'.
'tact' after 'in' is 'INTACT'.
(Other definitions for intact that I've seen before include "Undiminished" , "Unbroken and unimpaired" , "without flaw?" , "Unscathed" , "Unbroken - sound" .)