District attorney initially thinks I have a case (6)
I believe the answer is:
dative
'a case' is the definition.
(the dative case in grammar)
'district attorney initially thinks i have' is the wordplay.
'district attorney' becomes 'DA' (abbreviation).
'initially' says to take the initial letters.
'i have' becomes 'I've' (contraction of 'I have').
The first letter of 'thinks' is 't'.
'da'+'t'+'ive'='DATIVE'
(Other definitions for dative that I've seen before include "Grammatical case" , "'To' case (grammar)" .)