You can´t have a "clock" without one osclilator...
If you are using VHDL for Behavioral simulation
entity clk_2MHz is
Port (clk_2MHz : out std_logic
);
end clk_2Mz;
architecture Behavioral of clk_2k is
signal clk : std_logic := '0';
begin
clk <= not clk after 250 ns...