Testimony of European study in immorality (8)
I believe the answer is:
evidence
'testimony' is the definition.
(testimony is evidence given in court)
'european study in immorality' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'study' becomes 'den' (term for a study or office).
'in' means one lot of letters goes inside another.
'immorality' becomes 'vice' (vice can mean immoral behaviour).
'den' inserted into 'vice' is 'vidence'.
'e'+'vidence'='EVIDENCE'
'of' is the link.
(Other definitions for evidence that I've seen before include "Confirmation" , "Matters of fact put to a court" , "Information to help form a conclusion" , "The basis for belief or disbelief" , "Supporting facts" .)