I have to pursue unfinished appointment, in case (6)
I believe the answer is:
dative
'case' is the definition.
(the dative case in grammar)
'i have to pursue unfinished appointment' is the wordplay.
'i have' becomes 'I've' (contraction of 'I have').
'to pursue' is a charade indicator (letters next to each other).
'unfinished' means to remove the last letter.
'appointment' becomes 'date' (synonyms).
'date' with its final letter taken away is 'dat'.
'ive' after 'dat' is 'DATIVE'.
'in' is the link.
(Other definitions for dative that I've seen before include "'To' case (grammar)" , "Grammatical case" .)