"To defeat an incumbent" means to win against a person who currently holds a position or office, usually in an election. It refers to overthrowing the current officeholder and taking their place. Full definition
She said she was voting primarily to help defeat the incumbents. (newsday.com)
Mr. Losquadro, who defeated incumbent Democrat Marc Alessi in November, said that while he plans to meet with residents this summer to address their concerns and create a plan - of - action for next year, he believes some of his constituents won't be in his district for long. (suffolktimes.timesreview.com)
I do not think it is wrong to project freshman Democrats who defeated incumbent Republicans as vulnerable. (voices.washingtonpost.com)