Case, one vital bag held back (8)
I believe the answer is:
Here is my best explanation:
'case' is the definition.
(ablative is a kind of case)
'one vital bag held back' is the subsidiary indication.
'held' says the answer is hidden in the clue.
'back' says the letters should be written backwards.
'evitalba' is hidden in the letters of 'one vital bag'.
'evitalba' back-to-front is 'ablative'.
(Other definitions for ablative that I've seen before include "Latin case", "A case in grammar" and "Case in Latin denoting 'by' or 'with' the associated noun".)