Make an appointment about four in case (6)
I believe the answer is:
dative
'case' is the definition.
(dative case is a kind of case)
'make an appointment about four' is the wordplay.
'make an appointment' becomes 'date' (a date is a kind of appointment).
'about' means one lot of letters goes inside another.
'four' becomes 'iv' (roman numeral).
'date' placed around 'iv' is 'DATIVE'.
'in' acts as a link.
(Other definitions for dative that I've seen before include "Grammatical case" , "'To' case (grammar)" .)