Complex sentence: Jameson first attempted to enter politics at the 1890 general election (the first to be held for the Legislative Assembly), but lost to William Silas Pearse in the seat of North Fremantle.
Split into simpler sentences: Jameson first attempted to enter politics at the 1890 general election, the first to be held for the Legislative Assembly. He was defeated by William Silas Pearse in the seat of North Fremantle.