Time invested in a viable criminal case (8)
I believe the answer is:
ablative
'case' is the definition.
(type of grammatical case)
'time invested in a viable criminal' is the wordplay.
'time' becomes 't' (abbreviation).
'invested in' indicates putting letters inside.
'criminal' indicates anagramming the letters.
'a'+'viable'='aviable'
'aviable' with letters rearranged gives 'ablaive'.
't' going inside 'ablaive' is 'ABLATIVE'.
(Other definitions for ablative that I've seen before include "Case in Latin denoting 'by' or 'with' the associated noun" , "A case in grammar" .)