Case, one vital bag held back (8)
I believe the answer is:
ablative
'case' is the definition.
(ablative is a kind of case)
'one vital bag held back' is the wordplay.
'held' says the answer is hidden in the clue.
'back' is a reversal indicator.
'evitalba' is hidden within 'one vital bag'.
'evitalba' reversed gives 'ABLATIVE'.
(Other definitions for ablative that I've seen before include "A case in grammar" , "Case in Latin denoting 'by' or 'with' the associated noun" .)