A holy book ... solemn vow ... what else? Finally, something to combat evil (8)
I believe the answer is:
antidote
'something to combat evil' is the definition.
'a holy book solemn vow what else? finally' is the wordplay.
'holy book' becomes 'nt' (short for New Testament).
'solemn vow' becomes 'I do' (I do - vow said at weddings).
'finally' suggests the final letters.
The final letters of 'what else' are 'te'.
'a'+'nt'+'ido'+'te'='ANTIDOTE'
(Other definitions for antidote that I've seen before include "It couneracts poison" , "Counteracting medicine" , "Donate it to anti-poison agent" , "Agent counteracting a poison" , "Medicine countering a poison" .)