Fruit boxes drip in this case (6)
I believe the answer is:
dative
'this case' is the definition.
(the dative case in grammar)
'fruit boxes drip' is the wordplay.
'fruit' becomes 'date' (date is a kind of fruit).
'boxes' is an insertion indicator (the outside letters must 'box' inside letters in).
'drip' becomes 'IV' (intravenous drip).
'date' placed around 'iv' is 'DATIVE'.
'in' is the link.
(Other definitions for dative that I've seen before include "Grammatical case" , "'To' case (grammar)" .)