Maintained a voter originally made a mistake (7)
I believe the answer is:
averred
'maintained' is the definition.
(averring is a kind of maintaining)
'a voter originally made a mistake' is the wordplay.
'originally' says to take the initial letters.
'made a mistake' becomes 'erred' ('err' can be a synonym of 'make a mistake').
The initial letter of 'voter' is 'v'.
'a'+'v'+'erred'='AVERRED'
(Other definitions for averred that I've seen before include "confidently stated" , "Asserted to be the case" , "Alleged, asserted" , "Stated to be the case" , "Held" .)