Case of fruit, one very small inside (6)
I believe the answer is:
dative
'case' is the definition.
(the dative case in grammar)
'fruit one very small inside' is the wordplay.
'fruit' becomes 'date' (date is a kind of fruit).
'one' becomes 'i' (Roman numeral).
'very small' becomes 'v' (abbreviation; 'small' suggests an abbreviation).
'inside' means one lot of letters goes inside another.
'i'+'v'='iv'
'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" .)