Case of fruit four tucked into (6)
I believe the answer is:
dative
'case' is the definition.
(the dative case in grammar)
'fruit four tucked into' is the wordplay.
'fruit' becomes 'date' (date is a kind of fruit).
'four' becomes 'IV' (Roman numerals for four).
'tucked into' means one lot of letters goes inside another (I've seen this in other clues).
'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" .)