Add one vote? It's no use to anybody (8)
I believe the answer is:
appendix
'it's no use to anybody' is the definition.
(the appendix has no function)
'add one vote?' is the wordplay.
'add' becomes 'append' (synonyms).
'one' becomes 'i' (Roman numeral).
'vote?' becomes 'x' (how votes are marked).
'append'+'i'+'x'='APPENDIX'
(Other definitions for appendix that I've seen before include "sometimes removed?" , "Supplementary part" , "Extra section" , "Supplementary material at the back" , "It's useless in the body, but useful in a book" .)