Complex sentence: Clendon was the co-convenor of the Green Party from 2001 to 2004, so didn't contest the 2002 general elections because the Green Party constitution does not allow co-convenors to stand for parliament.
Split into simpler sentences: Clendon was the co-convenor of the Green Party from 2001 to 2004. He did not contest the 2002 general elections because the Green Party constitution bars co-convenors from standing for parliament.