Upset voter stumbles against ballot box (8)
I believe the answer is:
overturn
'upset' is the definition.
(synonyms)
'voter stumbles against ballot box' is the wordplay.
'stumbles' indicates anagramming the letters (I've seen 'stumbling' mean this).
'against' says to put letters next to each other.
'ballot box' becomes 'urn' (urn can mean a ballot box in some countries).
'voter' anagrammed gives 'overt'.
'overt'+'urn'='OVERTURN'
(Other definitions for overturn that I've seen before include "Topple or capsize" , "Reverse; upend" , "Reverses, a decision say" , "Reverse (legal ruling)" , "Reverse, rescind," .)