substitute
someone designated to succeed a (usually elected) person in office when the latter stops holding the office (through death, resignation or other reason)
en-US/ˈsʌbstɪˌtut/
en-GB/ˈsʌbstɪtjuːt/
someone designated to succeed a (usually elected) person in office when the latter stops holding the office (through death, resignation or other reason)