Try eg study in English depravity that's inadmissible (7,8)

'that's inadmissible' is the definition.
(I know that hearsay evidence is inadmissible)

'try eg study in english depravity' is the wordplay.
I cannot really understand how this works, but
'try' could be 'hear' (as in to try someone in court or hear their case) and 'hear' is present in the answer.
'eg' could be 'say' (both can mean 'for example') and 'say' is present in the answer.
'study' could be 'den' (term for a study or office) and 'den' is present in the answer.
'english' could be 'e' (abbreviation) and 'e' is found within the answer.
'depravity' could be 'vice' (moral depravity) and 'vice' is found within the leftover letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).