Testimony from key study probing immorality (8)
I believe the answer is:
evidence
'testimony' is the definition.
(testimony is evidence given in court)
'key study probing immorality' is the wordplay.
'key' becomes 'e' (musical key).
'study' becomes 'den' (term for a study or office).
'probing' indicates putting letters inside (probe can mean to make a hole in).
'immorality' becomes 'vice' (vice can mean immoral behaviour).
'den' placed inside 'vice' is 'vidence'.
'e'+'vidence'='EVIDENCE'
'from' is the link.
(Other definitions for evidence that I've seen before include "Confirmation" , "Facts and figures relevant to a case" , "Matters of fact put to a court" , "Indication" , "What scientists need" .)