American lawyer set to defend one English case (6)
I believe the answer is:
dative
'case' is the definition.
(the dative case in grammar)
'american lawyer set to defend one english' is the wordplay.
'american lawyer' becomes 'DA' (abbreviation for District Attorney).
'set' becomes 'TV' (TV set).
'to defend' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'english' becomes 'e' (abbreviation).
'tv' placed around 'i' is 'tiv'.
'da'+'tiv'+'e'='DATIVE'
(Other definitions for dative that I've seen before include "Grammatical case" , "'To' case (grammar)" .)