Case of fruit: four inside (6)
I believe the answer is:
dative
'case' is the definition.
(dative is a kind of case)
'fruit four inside' is the wordplay.
'fruit' becomes 'date' (date is a kind of fruit).
'four' becomes 'iv' (roman numeral).
'inside' means one lot of letters goes inside another.
'date' going around 'iv' is 'DATIVE'.
'of' is the link.
(Other definitions for dative that I've seen before include "'To' case (grammar)" , "Grammatical case" .)