Vow prisoner's facing time finally before case (10)
I believe the answer is:
consecrate
'vow' is the definition.
(I know that vow can be written as consecrate)
'prisoner's facing time finally before case' is the wordplay.
'prisoner's' becomes 'con's' (abbreviation for 'convict').
'facing' means one lot of letters go next to another.
'finally' indicates one should take the final letters.
'before' is a charade indicator (letters next to each other).
'case' becomes 'crate' (crating is a kind of casing).
The final letter of 'time' is 'e'.
'cons'+'e'+'crate'='CONSECRATE'
(Other definitions for consecrate that I've seen before include "Sanctify" , "giving cause to be respected" , "Bless" , "Make 18 [SACRED]" , "Make holy" .)